'Primer' is one of those mindf*cks that will leave you a) completely cold, or b) wanting to watch it again, now. This film is along the lines of 'Pi', 'Memento' or 'Donnie Darko', but infinitely more frustrating because the clues are oh so subtle...
The premise is straightforward enough: two suburban techies invent what appears to be a time machine and start experimenting with it. From there the story grows hyperbolically more complex, as the characters (and their doubles, triples?) start "reverse engineering the present" and getting into all the philosophical paradoxes associated with time travel. At one point the narrator concedes: "The permutations were endless."
So, I won't even pretend to be able to explain the whole plot -- if that's even possible. The internet discussions are still far from settling on even a definite timeline of events. (Examples of timeline theories: here and here.)
But aside from all the who-did-what-when questions, more interesting is seeing the characters struggle with something they don't understand, something that is "unknowable." This is best shown in the little things, like one of the characters asking:
Man, are you hungry? I haven't eaten since later this afternoon.
Or, more ominously, when one character's ear suddenly starts bleeding, or when they talk about their handwriting inexplicably deteriorating. Is it the machine?
They don't know. They can't know. 'Primer' is unknowable.