Basically the air plane gets sent back in time because of some weird worm hole and lands on donnies house. Because he had been sleep walking or w/e he escaped death. Then the rest of the movie takes place and he pieces together the whole time travel bit until at the end when the future catches up with the past and his mom and sister blow up on the plane and he decides to let it kill him this time because it would save everyone in his life the grief of having known him.

I dont even understand what Im talking about but really the whole story is about how donnie decides to in the end let himself die in the plane crash because it prevents a lot of the bad stuff that happened from happening.