D*MNIT Donnie Darko, last time.

[DamnthatDavid]

All I got from the movie was a overly religious aspect. Then watching the extended scenes confirmed it.

Everyone has a path to travel through time, predestined. The path is broken, time paradox is started.
Donnie states he was afraid of dying alone and he is atheist.
At the end, Donnie knows he is not alone, dies happy.

Or

Donnie himself managed to send the engine back in time, creating an alternate reality, so that he might save his Girlfriend and Frank.
Which in itself is a paradox, for if Frank never would of died, then he couldn't of worned Donnie in the future.

Answer: The Movie itself lives in a alternate demisions that plot holes are the norm.