The film is a lot of fun.  Bruce Campbell, whose Brisco County Jr. was the centerpiece of a wonderful comic adventure, and whose Autolocus, King of Thieves, is one of the most enjoyable recurring characters in the Hercules and Xena episodes, brings a similar comic smugness to Ash, the anti-hero of the movie.  Although certainly not for children, the light-hearted feel throughout makes it more of a farce (a good farce) than a horror flick.  Right at the beginning, Ash is pulled through a dimensional rift into the past; the action all takes place in the past, and at the end he returns to the twentieth century and his job at S-Mart.  Thus it is indeed a time travel movie worthy of our investigation.

The problem that has stymied my investigation repeats in my mind as if the response of some computer:  insufficient data.  At each turn in the examination, there are unanswered questions.  I thought that I might find the answers in the prequel, The Evil Dead (or The Evil Dead 2, which I have been told is a remake rather than a sequel, and that only the production quality is different).  I have not seen that film (in either version); yet by interrogating those who have seen them, I have concluded that the answers I need are not there, either.  Thus I am faced with several possible time lines, which I will present for your consideration.

To reconstruct the full story in its bare essentials, it begins in the late twentieth century.  An archaeologist otherwise unimportant to the film has discovered a book, The Book of the Dead (or Necronomicon), and left it in a cabin in the woods.  Ash and his girlfriend vacation at the cabin.  She discovers the book and reads an incantation aloud from it, unleashing large numbers of undead creatures who kill her immediately.  He spends the rest of the film fighting them off, until in the end he discovers that there is within the book another incantation which will banish the undead, and he saves the world by reading this incantation aloud.  Unfortunately, he was unaware that this would open a dimensional rift which would pull him into itself, so he failed to secure himself before performing the ritual.  The world is saved, but Ash is lost, sucked into the vortex to disappear.  End of first movie.

As this installment begins, these facts are reviewed in brief, and we are given Ash's fate.  Carried back in time, he is dropped into the middle of a medieval war.  Immediately taken prisoner by one side and accused of being part of the other, he is tentatively identified by a local wizard as the prophesied one who would save them from the greater evil they are fighting, the undead creatures who lurk here and there.  The details of this evil are not fully explored, but it is clear that some horrors are loose in the world.  They need the Book of the Dead to halt it; although the leaders are not easily persuaded, they agree to give him the task of retrieving the book from an evil cemetery, and persuade him to do so with the assurance that the book has the power to return him to his own time.  He does this, but not without waking armies of the dead.  Leading humanity to victory, he kisses that world good-bye and returns to his own time.

There are two distinct timelines.  In The Evil Dead, we are given the end of the first; in Army of Darkness, we see the beginning of the second.  At the end of Army of Darkness, it is suggested that time continues, and therefore we should seek an explanation which creates an N-jump (rather than an infinity loop, both illustrated on the introductory page to this site).  If on the facts we can extrapolate a scenario which allows the ending given, that should be preferred.

The prophecy cited in the second movie complicates the first timeline.  Although the details of this prediction are not known, it appears that it foresaw the arrival of a hero who would capture the book and lead them to victory over evil, and that the specifics of these events are not clear.  Since the release of the undead resulting from Ash's error in the litany when he recovers the book is not anticipated, but he is expected to save them from the undead when that occurs, the matter is uncertain:  was he originally predicted to lead them in such a battle, or was that expectation added because of his prophesied connection to the book?  However, in the first timeline the prophecy cannot be fulfilled by the arrival of Ash, because time must advance to the point at which he is born before he can return to the past to fulfill the prophecy.  We are left with three obvious alternatives.  Either the prophecy was not made in the first timeline, or having been made it was not fulfilled, or it was fulfilled by someone else.

If the prophecy was not made or not fulfilled, we have a more complex problem.  In the early information of the second timeline, we see that there is a problem related to the undead.  By the end of that picture the problem has been solved.  If we eliminate the prophecy, we introduce a back-story to cover this on at least one side--that is, either the first timeline has a story to explain how the undead problem was solved, or the second timeline has a story which tells how it was created.  However, there is another complication:  during Army of Darkness, the book is moved from the cemetery to the fortress, miles away.  Centuries later, the book is found by the archaeologist--but we don't know where it was found.  If it was never removed from the cemetery in the first time line, then it must have been found there--but in order for it to be found there in the second time line, it must have been replaced, and there seems no good reason for anyone to have done that.  But if it was found somewhere else, then there must be an explanation for how it reached its new location.  The extremely unlikely answer is that at some point the book was moved to the fortress for other reasons.  However, this is not the only solution for this problem.  A mid story could exist.  At some time between then and now, for reasons of his own, someone desired that book, researched its location, and discovered it.  Retrieving the book, he placed it elsewhere.  In this scenario, it doesn't matter whether our unknown party recovers the book from the cemetery in the first timeline and from the fortress in the second, as long as it is reasonable for him to have tracked the book to its resting place and recovered it from either location.  Thus the archaeologist recovers the book neither from the graveyard nor from the fortress, but from this unknown location.

I am overall dissatisfied with these solutions.  Although they are plausible, they require us to extrapolate a great deal--the prophecy and/or its fulfillment only exist in the second timeline; the book is moved by an unknown later party, who finds it in one place in the first timeline but another in the second; the undead problem does not exist in the first timeline.  It's too many necessary differences which are not explained by the time travel itself, but are needed to maintain the story.  There is an easier solution to this entire problem.  We may assume instead that the prophecy is made and in the first timeline fulfilled by someone else.

This is not so much a stretch as you might expect.  After all, the wizards and wisemen of the day are looking for a savior.  They have a promise that someone will arise who will have the ability to recover the book and so deliver them from evil.  Had Ash not arrived, they could easily have identified someone else as the promised deliverer.  And let's face it:  Ash did not do the job all that well.  Coughing instead of saying the final word in an incantation is hardly a reasonable approach to magic.  It is entirely possible that the original promised one was not at all as good a combatant as Ash--but that having properly pronounced the spell, he avoided the war.  It may be that fewer humans were killed fighting the undead, but then perhaps more were killed in the continuing battles with other human factions.

So we're ready to recreate our twin timelines.

In the past, there was a prediction about the release of evil, and the coming of a mysterious individual who would rescue them by stealing the Book of the Dead so that the magic to destroy the evil would be available.  (The concept of destroying evil magic with evil magic leaves me uneasy, but it is common enough in these stories, and I'll not argue it here.)  History advances to the critical moment, which we will identify as point A.  At this moment, the problem of evil is running strong against mankind, and the wizards are looking for the promised deliverer.  Ash does not arrive (he has not yet been born), but they fix their hopes on another, who enters the cemetery and recovers the book by correctly pronouncing the ritual.  The war with the undead army does not occur, but neither does the armistice with the neighboring humans.  All things move forward several centuries, until an archaeologist excavates the fortress and finds the mysterious book.  The events of the second movie occur, with the death of Ash's girlfriend and his own battles with evil.  He discovers that he can use the power of the book to destroy the evil, but by doing so is thrown back in time.  The point at which he performs the ritual is our point B, and the end of the first (A-B) timeline:  there is no future beyond that moment for which those events are history.

As Ash reaches the time of point A, his presence initiates the altered timeline.  The wizards and wisemen quickly latch on to him as the promised one, and so are no longer seeking someone for that role; so the original hero is missed, overlooked to this history.  Ash changes the story somewhat, in that his failure to properly perform the ritual begins the war, but his knowledge of the technologies of the future help him win it, and his early kindness to the leader of the other faction of humans brings that faction to his aid, defeating the undead and forging peace between the factions.  Ash leaves the past to return to his own time.  The book is left at the fortress as it was in the A-B line, and is discovered by the archaeologist and moved to the cabin for Ash's girlfriend to discover, beginning Ash's involvement in events (from his perspective) in the C-D segment.  At point D, he is pulled to the past to perform his actions in the C-D segment; meanwhile, he also returns from the past to find that history has survived, and he continues his life in the twentieth century--although having improperly pronounced yet another spell, he has again released at least a small contingent of the undead who are focused on eliminating him (a consequence which he seems to enjoy, as destroying them in public is good for his image).

There were many moments in this story where it could have gone awry; however, not having many of the details made it possible for us to extrapolate a scenario which accounts for all of the given facts including the continuation of the future.  As indicated, it is not the only possible reconstruction of the timeline, nor even the only reconstruction which saves the future; but it does seem the most plausible given the known information.