Our story begins at a Halloween party, and every time Noah travels to the past he arrives in bed on that Halloween morning.  The party replays several times, with a lot of incidental characters--the Hulk drunkenly spilling people's drinks, the doctor giving Jello® shots from a syringe, the AC/DC musician, the dunking donut girl, the couple making out by the pool--each of which fits into a puzzle that brings Noah to Avery, and they share the evening, going to the piano bar where he is employed as piano player, hopping into the antique photo booth which Avery loves because she says you can hear history inside it as it develops the pictures, heading back to her place where they eat the rest of her favorite cereal, and then when he thinks he's about to kiss the girl of his dreams he gets a hug and the discouraging statement that it's great to have a good guy friend.  The next day when Avery goes to the nearby convenience market to replenish her cereal, she instantly falls in love with tall handsome Ethan (Robbie Amell, familiar as Stephen Jameson in The Tomorrow People and Ronnie Raymond in The Flash).  Three years to the day after that meeting, Noah is getting drunk at Ethan and Avery's engagement party, and Avery's good friend Carrie (Shelley Hennig) gets stuck driving him home, but he actually goes to the piano bar where his best friend Max (Andrew Bachelor) is trying to take care of him.  Noah lands in the photo booth, drops in the quarter while wishing he could go back and change things so that Avery was with him instead of with Ethan, and that's where the original timeline ends.

The photo booth bit bothered me.  It was in some sense too much like the fortune teller in Big.  Of course, a camera was also central to the time travel in Time Lapse, but it was an entirely different application.  I had to persuade myself that the use of the photo booth was not really an issue, and was actually kind of clever, with the idea of it containing history and so able to move Noah back to a previous moment in history--not the moment he was in the photo booth before, but the moment to which he wanted to travel.

The time travel in the film is magical:  an antique artifact has properties that take the user to a point in his own past.  It still gives us headaches.

The second biggest headache is that when Noah travels to the past he replaces his own self, his consciousness and memories from the future taking control of his body in the past.  This happens in Peggy Sue Got Married, Donnie Darko, all of The Butterfly Effect films, Hot Tub Time Machine, About Time, and quite a few other films, and it always creates the same issues.  The first of those is, what happens to the consciousness that has been replaced?

Obviously, the night before Halloween some version of Noah went to bed.  The next morning, the Noah from the future awakens in that bed, in that body--but the only memories he has of "yesterday" are now three years old, or else memories of the engagement party now three years in the future.  What happened to the memories of the original Noah?

Then it becomes more complicated, giving us the biggest headache.  At some point, seemingly perhaps a day or two after his arrival, future Noah leaves the past body behind and returns to the morning of the engagement party (the original one, November 1--in at least one iteration of history he changes that date to the day before).  Arriving in the future, he has all the memories of the original history through which he lived and of the day or two he just spent in the past, and with each trip this information accumulates--but he has no memory of the intervening three years as they actually happened.  What happened to the version of himself who actually lived through those years?  And why can Noah remember a history that he has erased so that it never happened?

We have to abandon any hope that this might be a replacement theory story.  If it were, when Noah traveled to the past he would find himself already there; then when the departure time arrived his other self would have to leave to create an altered past or the original past would be restored.  That self would know nothing of the original history but what the original Noah told him.  Assuming that happened as we see in the movie, but for the duplicate self, we would have a sawtooth snap with an infinity loop termination--we'll hopefully get to that.

We do better with some variant of divergent dimension theory.  Somehow Noah goes back to the morning of Halloween and creates a new history diverging from the old one; the old history remains.  It might be that in that original history Noah entered the photo booth and vanished, never to be seen in that universe again; it might be that he took some pictures and exited, unaware that a divergent self was now creating a divergent history.  Of course, in the latter case after he had succeeded once, the version of him left in the old world would be very confused that the time travel didn't work this time, and would likely try again, presumably creating dozens of divergent histories as each attempt to use the machine would create a new history and a version of Noah still sitting in the old one trying to make the machine send him to the past.  But we don't see those worlds.  This explanation, though, still hits the problem of the move to the future.  If Noah leaves from Halloween night to three years later, then for three years he is missing from that world and does nothing--but the movie suggests that some other consciousness of Noah does everything and then is obliterated when he arrives in the future.  However, that same consciousness must have been suspended, at least, when he took over that body on Halloween, and in fact unless it gets future Noah's memories somehow, or is a prisoner observing events around it, it will have no memory of having met Avery at the party.

No sane theory of time can give us Noah replacing himself in the past then leaping to the altered future with complete memories of the original future and none of the altered future.

One scene irked me not because of the time travel elements but because of their presumed impact on a specific iteration of Noah, and because I am a musician.  In the version of history in which he gives up playing in the piano bar so he can become a wealthy businessman, he sits at the piano at his engagement party, and discovers that he cannot play at all.

I'm sorry, but it just doesn't work that way.  I can't help being certain that he would have gotten the first chord right, and then started stumbling over keys--it would have been very hard to get someone good enough to create the sound of someone who was that good and is rusty, but you don't lose that kind of ability in three years.  I don't think you lose it in thirty years, at least, not completely like that.  Yes, you get rusty, and you can't play like you did--but you can still play.  You'll miss notes, get wrong chords, maybe stop and miss a beat while you try to figure out what comes next.  That's completely different from not being able to play at all.  The way Noah approaches the keys in that scene is worse than someone who has never played.

What makes it worse is that under the notions of the movie, he played within the past few days.  This body didn't, and he might have trouble with coordinating his fingers to play the right notes at the right speed, but his mind just played that song, and it can tell the fingers where that first chord is, at the very least, even if they can't keep up from there.

So no to that scene.

I said that it gave consideration to some points that a lot of films miss, so it's time to praise these.  What Noah ultimately realizes is that in order for the world to be right everything has to happen at the Halloween party exactly as it did in the original history.  He has to wear his original costume, and the Hulk has to spill the drinks so that he can save Avery from that, and Dunkin' Donut girl, who happens to be Carrie, has to make out with Jello Shot Doctor so she'll get him out of her system, because those tiny changes in the past completely alter the future in major ways.  Of course, in restoring the original details of that party, Noah ensures that Avery will marry Ethan, and not him--but he's come to recognize that he and Avery would never really be happy together, even if he abandoned his piano playing and became an incredibly rich businessman.

We noticed in About Time that Mary was going to fall in love with someone right about that time, and Tim was in the right place at the right time but then wasn't because he changed history so she fell in love with someone else, but then again he was because he changed history again.  When We First Met suggests a different story:  Noah is fated to fall in love with Carrie, and Avery and Ethan were made for each other, and Noah's efforts to change that make everyone unhappy.  The lesson is to leave time alone, because what happens is always right, and any changes we make will ruin it.  It's a rather magical view of the world, suggesting that something good is in control, but overall it is a rather magical film, in the plot sense.  There is something indeed commendable about the idea that what happens is right, or at least is more right than anything we could make by changing it.

As mentioned, in the end Noah attempts to fix the past by putting everything back as it was.

There is a sense in which the obvious way to do that is not to travel to the past.  Unfortunately, for some reason (consistent perhaps with divergent dimension theory) the film assumes that once you change the past, it remains changed even if you don't travel back to change it.  Replacement theory requires that if you want the past to remain changed, you have to change it in a way that will cause your other self to make the same trip at the same time to make the same changes--nearly impossible and almost always winds up in an infinity loop.  However, in this case if Noah restores the past to the original history, that restored original history will include the version of himself who is hopelessly in love with Avery and goes back to the past to change it, starting the entire sequence afresh, so as I said, a sawtooth snap with an infinity loop termination.

In the divergent dimension theory version, Noah could go back and create yet another universe in which the world does come to what we see at the end of the movie--but of course that there are four (if I'm counting correctly) other universes in which everything is wrong.  Since we only see one universe, that doesn't mean the ending would not be possible--but for our already noted memories problems with the divergent dimension theory solution here.

It's difficult to write a conclusion here.  The film, as a time travel film, is a collection of failed time travel tropes put together to tell a somewhat clever story about a guy trying to change time so that he will wind up with the wrong girl, only to realize it and try to restore it so he will wind up with the right girl.  It has a happy ending, and makes a good point.  Just don't expect a wonderfully clever time travel story, just a clever love story.