One of the aspects that makes the movie complicated and confusing is the sheer number of time travel events it includes, and how they are interlaced.  This appears to be the complete list; each event is listed twice, with an arrival and a departure point.

• Sometime probably before June 13th, 2007, Mr. Landau, no given name given, arrives from 2029, not before August 20th; he brings with him the very important piggyback time traveler, the disk.
• About 8:00 A.M. of an unspecified date in what we presume to be 2012, henceforth to be called launch date, Randy Fraker arrives to attempt to prevent the upcoming disaster, having left from 11:16 P.M. that night.  We know it's morning because the camera feed on the security monitor when he reaches the office says 8:02 A.M.; when he calls Mr. Landau to tell him not to come, Landau is watching his mother deliver his younger self to school; and when he calls his own home and his mother answers the phone, she does not believe it's him because he is in the shower.
• Launch date, about 8:02 A.M., Jim, no last name given but the most talented computer engineer at M.I.T., arrives, having departed from roughly 12:05 A.M.  Had he arrived before Randy, he would be waiting in the office, because he knows that's where Randy is headed.  It's short-sighted on his part; he could have gotten there first.
• Launch date, 11:00 P.M. by the clock in the lab, Jim departs to midnight, the first test of the machine.
• Launch date, 11:00 P.M., time viewed on clock in the lab, Jim returns from about 12:04 A.M., a few seconds after the computer announces one minute to self-destruct, and reports the disaster an hour in the future.  He brings with him the camera given to him by Lewis Aberricki, which has the events recorded but has suffered some damage to the digitally stored data going through temporal transit.
• Launch date, about 11:08 P.M., about a minute after the clock in the lab says 11:07, William "Bill" Wishman, only person ever expelled thrice from M.I.T., travels to 11:50 P.M. to attempt to confirm Jim's account.
• Launch date, 11:11 P.M. by the clock in the lab, William's head returns, having departed at 11:54 PM, without his body.
• Launch date, 11:16 P.M. by the clock in the lab, Randy departs to 8:00 A.M. that morning to attempt to prevent the disaster.
• Launch date, 11:50 P.M. by the clock in the lab, Bill arrives attempting to confirm Jim's story, but does not see anything that looks like what was reported because it is only just beginning.  He gets in a fight with Jim, resulting in
• Launch date, about 11:54 P.M. by the lab clock, Bill's head departs to 11:11 P.M., leaving his body behind, when Landau activates the machine during the fight between Bill and Jim.
• Launch date, 12:00 A.M., Jim arrives from 11:00 P.M. in the first experiment, sees blood, bodies, and signs of a struggle in the lab, recognizes that a self-destruct sequence is down to four minutes, hears Gale screaming his name, finds Lewis dying in the hall, is given the camera, and rushes back to the lab so that he can return
• Launch date, at 12:04 A.M. by the lab clock, Jim departs for 11:00 P.M. to alert everyone to the disaster he has just witnessed and deliver the camera.
• Launch date, at 12:05 A.M. by the lab clock, between the time Jim departs for 11:00 P.M. (at 12:04) and the time the self-destruct blows (at 12:05), Jim departs for 8:02 A.M. that morning to prevent Randy from saving everyone.
• Sometime in 2029 not before August 20th, Mr. Landau departs from a facility in Destiny, Arizona belonging to the mysterious Max Devlin, carrying the disk back to 2007.

Also in the lab that night is Van Leng, armed security guard.  Laundau and Leng are the last two to reach the lab, at 10:15 P.M.  Laundau says they have been working on the project for two years, and the film was made in 2012, so we presume work started in 2010.

At 10:25 they power up the machine, causing a blackout in a forty-block area.  The self-destruct is activated at 11:40 P.M. by the lab clock.  Elevator power is restored at 11:58 P.M. by the lab clock.  At 12:01 the computer announces self-destruct in four minutes, which means it blows at 12:05, consistent with Landau's statement that it runs twenty-five minutes.

Alfred Hitchcock coined the term maguffin for that thing in the story that everyone is trying to get.  He said it doesn't matter what it is, as long as it is believable that everyone in the story wants it.  At first, we suspect that people are being killed because someone wants exclusive control of the time machine, but eventually we discover the disc.  It is a plastic digital storage medium initially in the briefcase of Mr. Landau, which contains stock prices running from 2017 to 2029.  It is how Mr. Landau has made his fortune (and why he has been investigated by the Securities and Exchange Commission nine times in less than eight years).  He brought it back from the future with him.

He claims that he took the disc from Max Devlin, a mysterious person whom he never met but who perfected the time machine, and that he fled to the past with it to make his fortune betting on sure things, as it were.  It is a clever idea, but it has at least three problems.

The first is the same sort of time loop that we saw with the watch in Somewhere in Time:  Landau took the disc from Devlin, but Devlin got it from Landau.  We tend to think of these discs as lasting forever, but they don't--they wear out, data gets corrupted, plastic warps, and this valuable object eventually crumbles to dust, reducing to useless long before that point.

Complicating it in this case, our second point, is that we know that the transport method is very hard on intricate structures.  Two of our time travelers are brain damaged by their trips, and the digital data in the digital camera is partly scrambled by being transported through time.  Why should this not also impact the microscopically stored data on a digital disc?  We are told by Landau that Devlin solved the brain damage problem, but then Devlin never made a time travel trip and might well simply have wanted Landau to believe that problem had been solved.  If it were only reduced, it would still impact some of the data on the disc, and every time that same disc makes that same trip, a little more of the data is corrupted by the journey.

This could be resolved if we assume that it's not the same disc, but that Devlin replaces the physical disc creating a new physical disc before each trip (similar to our suggestion in How Can I Change the Past?, but not by copying the potentially corrupted original disc).  We are not told either way, but it would be a simple problem to overlook, and fatal even to a fixed time theory of the events.

The other problem is a bit stickier.  Stock prices are ultimately a supply and demand question.  Sure, corporate announcements, discoveries, mergers, and other business matters impact them, but ultimately what determines the price of a share of stock is how much someone has to pay to get someone else to part with it.  That means that once Landau (or Devlin) starts buying stocks based on prices reported in the database from the future, those purchases start to impact those prices.  They also become a matter of shifting wealth, that money that originally went to someone else now goes to Landau, and investors whose purchases impacted the prices of some stocks are not making the same purchases because of losses that had originally been gains.  There is a ripple effect with every purchase, and thus the longer the list is in use, the less accurate it becomes.

In a fixed time story, this theoretically is not an issue because the purchases being made by the time traveler were already part of the original history.  This, though, becomes part of the strict determinism of fixed time, as the time traveler is not actually making intelligent decisions based on the data, but simply buying what he is destined to buy.

As we are watching the first time traveler depart, someone makes a comment about keeping his arms and legs inside the vortex so they won't be left behind.  It is a familiar concept with teleporting, the word splinch having been given to us by J. K. Rowling for the loss of body parts that get severed when the rest of the body is teleported.

The explanation includes the rather absurd suggestion that it has something to do with a black hole.  Ignore that explanation; black holes do not work in the ways necessary for the film.  It's just an attempt at a pseudo-scientific explanation for what's happening.  Gale's statement that they are looking at a "quark gluon" ("[speaks indistinct]" in the subtitles) is similarly pseudo-scientific, but at least it's not obviously wrong.

The warning is necessary so that we understand what happened when Randy's head arrives in the past; we are pretty much waiting for it to happen when we finally get to the moment in the future when it does.  That probably doesn't need further explication.

In working through the film, we observed a lot of little annoying problems.  Several of them are minor loops, small predestination paradoxes against the huge one.  Here's a quick look at them:

• Lewis says to Jim, "We kept filming like you said."  But Jim only says that because Lewis told him to say it.  That's pretty critical, because a lot depends on that film, including
• Gale saying to Jim and Lewis, "We found him [Landau] 'cause we saw we found him [on the video recording from the future]."  They might not have found him, and certainly would not have found him so soon, had they not seen the scene on the video telling them where they found him.  There's also another, shortly after that,
• The killer speaking to Laundau, "When the time comes, press the red button."  The killer only knows to tell Landau to do this because he knows that Landau in fact did it, and that he said he did it because it's what the killer told him to do.
• There's also the point when the killer says to Lewis, "Give this [camera] to Jim; it's your only chance."  So Lewis gives the camera to Jim because he was told to do so, based on the fact that Jim having the camera made it possible for them to survive as long as they did.
• We'll give one more, but it's a doozy:  Jim, the killer, has to persuade his younger self to become the killer.  Had he not done so, his younger self likely would not have become the killer, and none of this would have happened.

The characters also repeatedly make the same mistake in their logic.  Jim says it first, when he states "Nobody but us is down here, so whoever the killer is, it has to be one of us."  Gale says it next, "No one else has come down here so it has to be one of us."  Jim then says it again later, when he is insisting that Landau has to be the killer over Lewis' objections that the video says otherwise, "Look, look around, O.K.  I know I'm not the killer.  It's not you.  It's not Gale.  So that leaves him [Landau]."

What they are all missing is obvious:  it could be a time traveler from the future.  It happens that it is one of them, but let's be clear.  At the time they are discussing this, they know that someone sets the self-destruct, but they do not know that it was never cancelled.  If the facility survives, then tomorrow, or the next day, or next year, or ten years from now, someone could use that same time machine to travel back to the night of the first experiment and start killing people.  Even if the self-destruct ruins everything down there, given even a few decades someone could rebuild a new time machine and travel back to kill them, for reasons they could not begin to fathom.

It happens that, as in Triangle, the killer is one of them, but it is also the case (as in that movie) that the killer is a temporal duplicate, someone who traveled from the future to kill them.

This also gives us a very peculiar problem.  In the video Lewis watches, Jim says, as above:

"Look, look around, O.K.  I know I'm not the killer.  It's not you.  It's not Gale.  So that leaves him."

"Look around, O.K.  I know I'm not the killer.  It's not you.  It's not Gale.  So it must be him."

1. It's a continuity error, that they filmed the same scene twice, and the actor changed the line, and either no one caught it or they rather foolishly thought no one would.  It's a particularly unfortunate error, because it's pretty obvious that they set up the scene once, and filmed it several times from different angles to get it right, but then in editing they used one take for the scene itself and a different take for the preview version of the scene.  It's really very shoddy, if it's a mistake.
2. It's not a continuity error, but an intentional difference.  If it is, then we don't have a fixed time story at all, because in fixed time (as Lewis observes more than once) they would all say exactly the same thing consistently.  Jim phrases his statement differently.  It's not enough to cause a paradox under replacement theory, but it is enough to argue that this is not fixed time.

It might be argued that what the film intended to show was consistent with a fixed time story.  However, what was actually on the screen is not, so we are forced to seek a replacement theory solution.

Another minor problem, which I will pose as a question:  where is the voice of the self-destruct system during the climactic argument between the two Jims?  We hear it before their argument, when the killer launches the self destruct; we hear it several times in the last five minutes.  Yet during the entire several minutes of arguing and fighting right next to the console running the countdown, the voice is silent.  Probably they didn't want the problems of tracking the time against the argument and dealing with the interruptions to the dialogue of an intense scene; otherwise we're left thinking that it's a rather strange design for the self-destruct system.

Little problems like this riddle the story.  When Landau sends William to the future, the activation of the time machine triggers an announcement throughout the lab that brings everyone running; but that's the only time we hear that announcement, and if it could be disabled certainly Landau would have disabled it for that trip.  To make the first trip, they have a major charging process that blacks out a forty block area, but they then make five more trips without similar problems; if they were charging capacitors for this, they had all month to do it and could have avoided the abrupt massive power draw; if they needed the power from the grid for the trip, they needed it every time, not just the first time.  So we have a lot of annoying little problems that get swept under the rug in an effort to create greater drama.  These are all things that happen when they are necessary to the plot, but they ring of Roger Rabbit saying that he could only escape from the handcuffs when it would be funny.

The N.S.A. is waiting outside the lab, and calls for backup when the power fails.  The team leader tells her green partner that they are there because of a phone call made by Randy, the call we saw him make to tell Landau not to come.  The awkward questions are, why does the N.S.A. have this, and why did Landau apparently not receive it?

It is obvious that Landau, paranoid as he is about government in general and the N.S.A. in particular, did not give them the message.  He might not have listened to it, but turned his attention to whatever busy-ness comprised his day.  He might have written it off as a practical joke.  He might have called someone in the security office of the lab and asked if anything unusual had happened, gotten a negative reply, and decided it was nothing.  Had he taken it seriously, he might not have come--or he might have, because the project is that important to him.  He would not, however, under any circumstances alert the authorities to the message he received.  The stakes are too high, and he does not want them to know what he's doing.

However, we know that the N.S.A. is investigating the lab.  They have inserted an undercover operative.  It is likely, then, that they have a tap on the phone, and recorded the call when Randy made it.  They thus got it independently of Landau, and knew there was trouble whether or not he did.

So to unravel this, lets see if we can place the first trip sequentially.

The obvious one is the one in which Jim goes forward one hour.  Everyone thinks that's the first trip, and that's the first departure point in our temporal sequence.  However, Jim's departure is entirely dependent on the presence of Mr. Landau, guiding and funding the project, and therefore Mr. Landau's trip from 2029 to 2007 must precede it.

So perhaps that's our first trip.  However, Mr. Landau made his trip in a time machine funded by Max Devlin, whose wealth came from his uncanny prescience of stock prices due to the disk which he apparently compiled in the future and sent to the past.  Further, Mr. Landau asserts that Devlin's machine only becomes possible because of the success of their experiments in 2012, so Jim's trip has to happen before Mr. Landau's.

That's pretty much fatal to the entire story.  We can work through the other trips and find plenty of mutual dependencies.  Jim travels back to kill Randy because Randy already traveled back to stop the killing, but the killing wouldn't have started without Jim traveling back to kill Randy.  Jim's trip back to warn everyone at eleven only happens because he traveled forward, but Will's trip forward is also due to Jim's return trip, and the return of Will's head is dependent on his trip forward.  Every one of these trips through time is dependent on at least one other trip through time happening before it, so not one of them can be the first.

That means for this to have happened at all there has to be a different original history, that someone did something that started the entire story which was then replaced by someone doing something similar enough to cause the same chain of events.  Where did that happen?

The best guess is that in 2029 someone invented time travel--very possibly someone we never meet, someone never named.  It's not even really unlikely.  After all, we have previously spoken of inventions being "ripe".  Alexander Graham Bell is remembered for inventing the telephone because he reached the patent office with his design half an hour before his nearest competitor.  If the technology exists by 2012 to discover a way to travel through time, even it requires help from the future, someone is going to figure it out by 2029.  After all, we've got a couple guys working in their garage (Primer), secret projects funded by major corporations (Synchronicity, Terminator Genisys), crazy scientists working in their own homes (Back to the Future, Safety Not Guaranteed), not to mention the wealth of accidental discoveries and visits from future time travelers bringing machines back to us.  Someone is bound to have a breakthrough by then.

Nor is it a big leap to suppose that whoever first invented such a machine would get the idea of taking stock exchange prices back to make himself rich.  You don't even have to slog through Primer to get the idea; someone did it in TimeCop, and it's a lot safer than betting on races through a bookie (Time Lapse) and probably more reliable than a sports almanac (Back to the Future III).  So someone compiles the disc and finds a way to send it back to himself.

But wait--isn't the 2012 experiment necessary for the 2029 machine to succeed?  We have that from Mr. Landau, but is it true?  He has it from Max Devlin, who apparently led him to believe it would be necessary for him to build a time machine in 2012 in order for the 2029 machine to exist.  What, though, did Max Devlin get from that 2012 experiment?  The explosion destroyed the lab; there was nothing left of the equipment or the research.  Sure, Max went to M.I.T., but if Max was recruited after the first year of school, how much did she learn?  She saw the equipment, she saw that it was possible, she saw the formulas but didn't take them with her.  She knows a little bit about how they did it (she did know that the bubble had something to do with quark gluons, but that's not a lot to know).  What did she really get from that project?

She got the disc.

O.K., she got the knowledge that time travel was possible, and that by 2029 someone would solve the problems and make it possible for someone to travel to the past.  That put her on the track to do that.  That also means she replaces the original creator of the time machine and becomes the first person to successfully build such a machine, so that it is her machine that sends the disk to the past from 2029.  But mostly, she needed that project to come into being so that she would get the disc.

So what about Mr. Landau?

Someone built a time machine.  Their first inclination is to send someone back with stock prices.

I'm willing to bet that the original Mr. Landau was much better informed than the one we see.  He was the chosen time traveler, given the stock numbers.  The original inventor of the machine chose not to be the guinea pig, for whatever reason--it might have been concern about brain damage, or it might be that he was simply too old to travel back and expect to live long enough to make a difference.  It's a simple matter for his employee to hop back to 2007, begin investing, deposit funds in accounts set up in the name of his boss, and make the boss rich in the future.  He might also have suggested that Landau should start work on a time machine, so that they can use the early research to complete the project sooner.

As Devlin's project is replacing the original one, she knows that Landau was the person who came from the future with the disc.  She has to create her own version of the disc, but she also has to derail the original project and bring Landau to her time machine and her disc.  As part of that she creates the notion that the 2012 experiments are necessary for the existence of the 2029 time machine (as noted, they aren't really, although we would have an infinity loop without them).

This version of Landau knows far less, but he knows enough to raise the money and start the experiment, and so we have our 2012 experiment.

That means our first trip to the past has to be Mr. Landau, working for our lost original inventor, carrying a disc back to 2007 and starting to amass the money needed to launch the 2012 time travel experiment.

Landau puts together a team to work on time travel.  It is at least a bit improbable that the original Landau would have put together the same team as his later version, but then, it's not so unlikely when we consider the details.  Our four key people are all top of their class at Massachusetts Institute of Technology (and probably only Cal Tech would argue that they're not the best), each in a different area--Jim in computer engineering, Lewis in computer programming, Randy in astrophysics, and we're not told exactly what Bill's field is.  They each have reasons for wanting to be part of something as big as this.  That Landau would both attract and hire the same quartet is not as improbable as it might sound, because he wants the best, and they arguably are the best.  They also know each other, having encountered each other at school together.

Nor is it unlikely that the Landau who attracted the attention of the N.S.A. in later iterations would have done so originally, between his prescient ability to pick winning stocks that caught the attention of the S.E.C. so frequently and his highly secretive program recruiting scientists to work in a secret lab, and they might well look for someone who could infiltrate the program.  A young woman drawn also from M.I.T. who could attach herself to one of the men would be the best pick, so Gale winds up with that assignment.

There is a question of what all these people did in the original history, before Landau traveled back and recruited them.  That question is particularly significant, because they probably would have worked for companies that made significant strides based on whatever they were doing--aerospace engineering, Silicon Valley projects, major technology firms of one type or another--and that means that moving them out of their original history into what they're doing now has a good chance of changing stock prices particularly among leading technology stocks.  That's not a disaster, though.  The original Landau knows that the information he has is subject to change without notice, as they say, and the disc compiled subsequently by Devlin will have been based on the world in which these changes have already been incorporated.

That matters, because three of those six people--Landau, Gale, and the killer--absolutely must be involved in this time travel experiment, and it must both succeed and go horribly wrong.

It's not easy to get there, but we're on our way.

We have established that the original first trip had to be one in which someone, probably Landau, traveled from 2029 to 2007 with stock prices, and that this sets up our present time travel experiment.  That trip will morph, as events occuring in its timelines shift, but we always have a Landau traveling back with stock prices, or time collapses.  All of our other time trips are dependent on the success of the one in which someone goes from 11:00 to midnight on our launch date, and everything points to that being Jim.

When Jim arrives at midnight in the first iteration of these events, he is greeted by the team, ready to celebrate the first successful step through time.  No one is dead; there is no gore, no mess, no screaming girl in the hallway.  Everything is successful, and it looks as if our story ends.  Of course, we know that it doesn't end here.  At some point the killer has to travel back and start killing people, but he hasn't done that yet, and we have to figure out when and why he does.

The fact that no one has been killed is only a piece of what hasn't happened in the skipped hour.  Since no one was being killed, no one broke into Landau's locker to investigate him, no one found the disc, and no one knows he's a time traveler.  There's also no camera to analyze.  These people are doing very different things, and probably not bothering to record most of them.

However, the discussion they had about changing the time traveler from Bill to Jim was held before Jim made the trip, and was recorded.  All of that is part of the record.  At this point, Jim will probably look over the record to see what he missed, and find that part of the tape.  This gives him the knowledge of their attitude toward him.  That might have been amplified in his absence, although it might or might not have been recorded, but the attitude of the other three members of the crew might be turning more strongly against him in his absence.  For one thing, it is at least plausible that they don't want history to record him as the first time traveler--the Neil Armstrong of their moon race.  They might be discussing whether to have the records say that Bill made the trip.  They'll also want to downplay his involvement--already they called him "glorified tech support".  They want the credit; they want him forgotten.  He might become aware of these aspects.  Further, he has already suffered some brain damage from that first trip--not as much as he has by the end of the film, but enough that he's going to be more sensitive to emotional swings.  Further, at one point Landau comments that he's surprised Jim hadn't realized that he had suffered brain damage from the trip.  Having suffered less brain damage, Jim might realize that he had suffered some, and guessed that Landau knew or should have recognized the possibility.  Even if he doesn't rationally think Landau knew, he would blame the man for his condition--and let's face it, his entire future is built on his intellect, his superior skill in computers.  They took that away from him, and want to take from him the success that caused the loss.

It is at least a plausible reason for him to decide to kill everyone.  It's also likely that he would recognize the need to do so by traveling back to the past--not so as to undo his trip to the future, but so that he can kill people and they won't know who is doing it because they will believe he is not in the building.

As long as he doesn't leave before 12:05 A.M., we have a workable scenario--and since it's going to take time for him to process this and the self-destruct countdown isn't running, he's not going to be in a rush.

The story does not happen unless the Jim who made the first successful trip forward through time goes back to some point in the past and starts killing people.  The question is, when?

We are lulled into a false sense of events by the leap made in the film.  We see Mr. Landau ignoring the phone call from Randy, and we see Randy in the shower in the morning, and the next thing we see is government agents on stakeout that night as William, Lewis, Randy, Jim, and Gale arrive.  Then Mr. Landau arrives, and when he reaches the lab it is quarter after ten at night.  It suggests to our minds that these are the next events which happen.

However, why is Randy in the shower at eight in the morning if he doesn't have to be at work for at least twelve hours?  Is he holding a day job somewhere? Not likely--Mr. Landau can certainly afford to pay competitive wages.  We see them all arrive late that night, but where were they all day?

The obvious answer is that they were here, at least some of them, in the lab putting the final touches on everything.  At some point they left for dinner, got some rest, and returned; maybe William picked up Lewis.

That matters, because it means that the Jim who decides to travel back in time has something of a connundrum.  He daren't arrive while people are there, or even while they might be there, because they'll see him arrive.  However, they must start arriving around nine in the morning, and he can't be certain about when they all leave for dinner.  He can't really rely on his memory, because he knows he's going to change history when he goes back, and that might mean they find him in the lab, or that they discover that he arrived.  Thus he aims for early enough that he expects no one will be there--sometime enough before nine in the morning that he has plenty of time to figure out what he's doing.

He doesn't want to kill anyone before they've sent him to the future, because that would undo his knowledge of events; thus he has to avoid being discovered.  There aren't a lot of places to hide, but the server room seems to have an abundance of nooks and crannies, so he might be there.  A stray comment from Gale about the boys' locker room suggests that there is also a girls' locker room, which only she uses, so he might hide there.

Once he knows he has been safely sent to the future, he'll start killing people, beginning with Van Leng because the man is armed and poses the most serious threat to his success.  After that he probably starts killing the others.  He can't kill Bill before 11:11, and he can't kill Randy before 11:16, but he will be looking for opportunities to kill everyone but Gale.

At some point, he realizes that he's going to be the obvious suspect as the only survivor, so he has to destroy the evidence; if he and Gale are the only two who make it out alive when the self-destruct detonates, she will be the only one who knows, and if he's careful she won't know that he was the killer.  So he sets the self-destruct for five minutes after eleven.  He hasn't realized that that will undo his trip to the past.  That would normally be a disaster, but he has inadvertently managed to correct the mistake in advance.

This is also the first history in which Mr. Landau is "outed"--because people are dying and Landau is hiding, they break open his locker and open his briefcase, find the disc and discover his secret.  The Maguffin is now in play--but the killer knows nothing about it at this point, because in the history from which he came, it was never discovered.

Having leapt to the future and subsequently learned that his peers are going to dump him, Jim traveled back to early that morning and hid.  He can't start killing people until after his younger self is sent to the future, so he waits; then he needs opportunity.  Because Jim did not return from the future this time, no one is worried about everyone dying, and their movements are entirely different, unknown to us.  Thus Jim has to find an opportunity to kill Van first--as mentioned, Van has a gun.

His next target might well be Randy.  Gale tells us that Jim liked Randy; the betrayal that hurts most is always the one from the person we thought the best friend.  He needs to catch Randy alone; after that he kills William, who poses the greatest threat and is the nastiest of the bunch.  Killing the kid in the wheelchair is easy, and the old man isn't likely to put up too much of a fight.

Some of those bodies wind up in the lab itself.  This time when Jim comes forward, he sees those bodies, recognizes the self-destruct sequence is running, finds Lewis in the hall, rushes back to the lab, resets the machine, and sends himself back to eleven o'clock to warn everyone of the coming disaster.

You are undoubtedly thinking yes, but if Jim goes back to 11 now, then he's not going to be here to make the trip back to 8 in the morning a few minutes from now.  Ah, but this is where it gets tricky.  Let's suppose that our original time traveling Jim actually worked out what was happening by ten after twelve, and went back to the morning to begin killing people.  Time didn't get that far this time around, so that version of ten after twelve has not been erased.  That means Jim the killer is still arriving around eight in the morning, and looking for a place to hide.  Since he was not waiting in the lab when his doppelganger arrived, he does not know that his doppelganger went back to eleven, and he repeats his same strategy.  This time, however, things are different.  Jim's arrival at eleven with the report of the horror in the future has everyone on edge.  Van suits up to fight whoever it is, but is attacked before he can do more than that.  At this point, it is agreed to send William to the future to investigate Jim's story.

We don't know what happens to William, but we do know that no part of him returns:  history has to advance through the forty-two minutes of William's forward trip before William can actually arrive there.  They should have considered this, but are so fixated on the notion that this is fixed time that it doesn't occur to them that it might be otherwise.  The Jim who left from the further future kills Randy, but not in the lab; Jim and Mr. Landau are in the lab when William arrives, and Jim thinks Landau is the killer and is threatening him with a gun.  There is an argument much like the one in the movie, and it turns into a brawl.

At some point, Landau decides to push the red button.  He probably was not told to do this in this history--no version of Jim would have that idea at this point, because it hasn't yet happened.  However, his life has been threatened by Jim, and he has no particular reason to believe that Gale and William are going to believe he is not the killer, so perhaps when he saw both of them leaning into the vortex space, he thought he could kill Jim, and William might be collateral damage.  Unfortunately, he killed William and not Jim, who leaned back out of the area when he heard it activating.

Time rewinds forty-three minutes from 11:54 to 11:11, and Bill's head arrives on the platform, freshly severed and still twitching.  Killer Jim doesn't have to kill Bill this time, because Bill is already dead.  However, before he can kill Randy, Randy agrees to attempt to warn them by going back to eight in the morning.

This one is tricky.  Randy has been fatalistic to this point, believing that they can't escape their deaths; what persuades him is the video from the future that includes an image of him, dead.  That brings us to another loop.

In the version of events between eleven and midnight that we see, a tremendous amount of it is at least influenced by the fact that right after midnight Lewis gave Jim a digital video camera on which he had been recording all of the events of the hour.  Jim takes it back, and data is scrambled in the vortex, but he gradually recovers images and audio from which they start to put together what is going to happen.  However, Lewis specifically states that he recorded everything because when Jim brought the camera back he told him to, and Jim tells Lewis to record everything because Lewis said he should say that, so whose idea was it?

It probably doesn't matter.  Probably Lewis originally recorded considerably less information, but told Jim to take the camera back and see whether there was anything on it that would be helpful.  Probably as they looked at how little they had, someone said they wished Lewis had recorded more, so Lewis started recording more.  When he then gives the camera to Jim, he tells him to say that they should record everything, which Jim does, so Lewis does, and the loop stabilizes

The other problem is that we have a moment when the killer retrieves the camera from where Lewis dropped it and tells him to give it to Jim to take back, as that is their only hope.  It's not clear why he said that; it's also not clear whether Lewis gives the camera to Jim because the killer told him to.  However, that version of the killer doesn't exist yet, so that's not the motivation at this point, and once that version of the killer is there to say that, Jim already knows that he had the camera in the past and got useful information from it, so the statement becomes irrelevant.

The camera will have an image of Randy dead, and while it won't be the same image it will be sufficient to motivate him to travel back to the past to try to save everyone.

Randy arrives in the past, around eight in the morning, and as soon as he manages to get oriented he heads for the security office and the telephone.

As you recall, the killer already came back to sometime early this morning, because he needed to be here before people started arriving for work.  When Randy arrives via time machine, though, this surprises him--he was unaware that anyone had traveled to the past after he left for the past, and if he saw the original video Randy was in it during that hour.  However, in the silence of the underground lab he will hear Randy arrive, and will figure out what he's doing quickly enough, and stop him.  He hides the body in the locker, and hides himself awaiting the others, repeating what he did before, except that Randy is already dead and William leapt to the future before he got him.

Randy's trip stabilizes, because not having prevented the murders he does not prevent his own trip to attempt to stop them.  Everything has settled by the time the first Jim reaches midnight, and rushes back.

There is also at midnight two other Jims, the one who has been killing people and the one who has just lived through the hour in which everyone is being killed.  He is not the same Jim as the one who made the trip and started killing people, because that Jim did not live through that disaster, so he has that new information.  During that hour, he also saw the discussion in which his peers disdained him, although he did not see the discussion (which now does not happen) in which they agreed to write him out of his place in the history.  He also has two other significant bits of new information.  One is that he knows about the existence of the disc, and that if he can manage to escape with it he can make himself enormously wealthy.  The other is a bit more complicated.

He knows that Randy was shot sometime in the morning when he went back to try to warn everyone, and that William was beheaded by the time machine, and that Van, Lewis, and Landau were also killed.  Of the seven people who entered the lab, five are dead; the only survivors are Gale and himself.  On ordinary murder mystery logic, if he knows he is not the killer, she must be; but he doesn't believe she's the killer; she's been outed as the N.S.A. agent.  He realizes that either he is the killer, or he can be the killer, and if he is not the killer then he's a target.  So his best chance to survive at this point is to travel back to that morning and become the killer.  He knows that Randy was headed for eight o'clock, so he makes his trip to meet him there.  He does it just in time to beat the self-destruct, at 12:05.

Yes, there are now two killer Jims in the compound at eight in the morning--the first one having left from the original history not before 12:05 and the one who just left at 12:05.  However, that second one has come specifically to kill Randy; the first one was unaware that Randy was going to be there, and now is twice surprised when a killer dressed as he is arrives and kills Randy.  This Jim goes into hiding, not knowing who the killer could be.  At this point, he probably thinks his best chance is to let the killer do all the work, then surprise the killer and kill him, become the hero, leave hopefully with the girl and the disc, and live happily ever after.  He killed William, but it wasn't really intentional--arguably it was Landau who killed William.

The plan probably would have worked, but before he gets to the place where he attacks the killer he is shocked to learn that the killer is he.  He can't figure out how that can be, unless for some reason he failed to kill everyone the first time and went back to try again.  He watches events unfold as his other selves argue, but he doesn't know what to do so he does nothing.  He is killed by the self-destruct, and no one ever knows that he is there.

At five minutes after midnight, the lab is destroyed, the original killer killed in the blast.  We know, too, that the replacement killer is also killed in the blast, and that Gale is the only survivor.  However, we have a problem.  The original killer went to the past not before five minutes after midnight, and now the time machine is gone and the person who made that trip does not exist.  That means that that person will not leave for the past, and will not arrive in the past.  Doesn't that create an infinity loop?

What generally creates an infinity loop is that each of two histories cause the other.  Lewis' desire to prevent his accident is a good example drawn from the film:  he is in a crash because he was driving drunk, so he wants to build a time machine so that he can go back to that night and prevent himself from driving drunk.  Were he to succeed, he would eliminate his reason for building the time machine, and even if he were tapped for the program he would not think to travel to the past to prevent an accident that never happened, so he wouldn't go, and the accident would happen, leading to him making the trip to prevent it.  The two histories cause each other.

Here we have the time traveler setting the self-destruct which will destroy the time machine before he makes his trip to the past.  He has prevented his own trip.  However, he has also set in motion events which cause his doppelganger to make a trip to the past sooner than the one he made, and his doppelganger similarly sets the self-destruct (possibly only because it was running when he reached the future originally), and so the original killer is prevented from making the trip entirely independently of his own actions, every necessary action he originally performed now being done by someone else.  He drops out of history entirely, never having existed except in the sense that he existed in a world that has been erased and replaced.

In the end, then, it is not impossible to arrive at the story on the screen through replacement theory.  It is an extremely complex and convoluted interaction of temporal events, one of which is necessary to launch several of the others but which itself is erased such that it never happens.  We've seen movies that were more difficult to unravel, but this was a tough one.

Bear in mind that at several points this story could have become impossible.  There might be other resolutions, but the one presented seems the most plausible, and many of the proposed versions of history must have happened very much as stated in order for the events on the screen to become the ultimate version of history.

So although it was a difficult temporal mess, it appears that Paradox is at least possible.