system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Donnie Darko

It must have been 10 years since I first watched it. Every time I watch it I spend the next week thinking about, then finally thinking I get it only to realize I don't.

What happens in Donnie Darko?
gpt
I assume you don't really need a plot explanation, more of a meaning explanation, right?

There's a boy named Donnie who most people think has trouble telling the difference between what's real and what's pretend. He has an imaginary friend who wears a bunny suit, named Frank. Frank takes him for a walk. Frank tells him that the world is going to end. When Donnie goes home it turns out that going on a walk saved his life. By not dying, an alternate reality was made. 
His doctor gives him medicine to help Donnie tell the difference between real and pretend but instead of getting better, Donnie sees more make believe things. Frank encourages Donnie to do some things that are bad but those bad things turn out to be good for many people. When Donnie did those bad things he only knew about the bad consequences. Frank takes the mask off his bunny suit and it looks like someone hurt his face really bad. Frank tells Donnie that time travel exists.
On Halloween, Donnie is walking with a friend and his friend gets hit by a car. The driver who gets out looks like Frank but he doesn't know Donnie and his face isn't hurt. Donnie is sad about his friend being hit by a car so he hurts Frank's face.
 Donnie knows that today is the day Frank said the world would end. Donnie is sad about all the bad things that happened since going on that walk with Frank, so Donnie sends an airplane back in time to when he went on his walk with Frank. The difference is that this time Donnie didn't go on a walk with Frank. Instead, Donnie dies. Because of this, the alternate reality with all the bad things is avoided.

Hope that clears it up.