I decided that next Halloween, I'm gonna get the costume of frank. But got me thinking, does anyone actually understand what happens at the end?

This is probably the best explanation I've seen for the ending:
http://www.donniedarko.org.uk/explanation/

this is from the director, richard kelly:

at midnight on October 2 a Tangent Universe spins off the Primary Universe at the moment when Donnie is called out by Frank for the first time, immediately before the appearance of the Artifact, the errant jet engine. This has started a causal loop. Tangent Universes are inherently unstable and will collapse in less than six weeks, taking the Primary Universe with it if not closed off (i.e. corrected). Closing the Tangent Universe is the duty of the Living Receiver, Donnie, who wields certain supernatural powers that can help him in this task.


Those who have died/will die within the Tangent Universe are the Manipulated Dead (Frank and Gretchen). They are also given certain -- but varying -- powers in that they subtly understand what is happening and have the ability to contact and influence the Living Receiver via the Fourth Dimensional Construct (water). All others within the orbit of the Living Receiver are the Manipulated Living (Ms. Pomeroy, Mr. Monnitoff), subconsciously drawn to push him towards his destiny to close the Tangent Universe and, according to the Philosophy of Time Travel, die by the Artifact.


There are two "Franks" in the story: the living boyfriend of Donnie’s sister Elizabeth, and the (same?) dead Frank who appears to Donnie as a premonition from the future in the disturbing rabbit suit. [The second Frank is dead (or undead), for at the end of the film he is killed by Donnie]. Dead Frank, as the most powerful of the Manipulated Dead, knows that Donnie is the only person who can close off the Tangent Universe, and has the ability to travel through time in order to guide him. If Donnie dies on October 2, or if Donnie cannot complete his quest, the world is doomed, for the paradoxical causal loop (caused by the portal) wil tear the Primary Universe apart.

so donnie never traveled in time, frank and donnie created the causality loop and the tangent universe when frank woke donnie up on Oct. 2

At least the director actually knows, and is willing to explain it. I've seen some give the answer "That's up to the viewer", to me that equals "I have no idea what it means, or why I tacked that ending on."

I've always considered the ending intentionally ambiguous.

Essentially, I think one of two things happened -

1) Donnie is actually somewhat psychotic and he's an unreliable narrator, so we never really know if he stopped his medication, if the medication induced greater hallucinations, or if he ever actually met the girl.

2) It's a straight time-travel paradox. Donnie is allowed to see the value of life but only through his own death.

So... the second would kinda take after 12 Monkeys. Something always has been and always will.

the only thing i really don't understand about kellys version is why frank called donnie out anyway... if donnie dies the first time, then there wouldn't be any TU or CL...

what was the reason frank called donnie out in the first place?

and how did frank know? was frank the cause or donnie? now i'm confused... maybe plastic is right, and there is no answer..

Well thanks, good efforts, clearer now, but still don't fully understand.

Anyhoo, great movie!