Donnie Darko is indeed not hard to get. The story begins just before the universe splits, he is in a parallel universe that is unstable, because a copy of an artifact already in the universe exists (The Jet Engine). Frank is someone whom already died in the parallel universe before, thus is able to return to guide Donnie to do what he must do: return the artifact back to its original universe, and restore time.

In case you didn’t catch on, Donnie always has a sense of clairvoyance when it comes to the events in the movie. This is because they have happened, many times, and while Donnie can slightly remember what’s happening, every time still Frank cannot convince Donnie to save the universe (And essentially kill himself), he instead must send him back in time to the beginning; This only causes him to lose most of his progress and try again.

The difference in the version you see is that Donnie falls for Gretchen due to the criminal acts he commits and the effects they have on the character interactions.

Need I say more?