Donnie sleepwalks a lot. People think he is weird. He takes pills. He upsets his family.

One night he follows a guy in a bunny suit in the middle of the night. The guy tells him the world is going to end. His name is Frank.
That same night. a jet engine crashes into Donnie's room. Had he been there, it would have killed him.

Donnie continues to frustrate his family, but he intrigues two teachers at his school. Gretchen Ross is new at school and thinks Donnie is cute. He asks her out. They are dating.

The class reads a book by Graham Greene about creation out of destruction. Frank tells Donnie he should flood the school by breaking open the water main (which is what happens in the book).

The PTA does not approve of this. They fire the teacher that had them read the story.

Donnie runs into this old lady who checks her mail every day, but never receives any. The kids call her Grandma Death. Grandma Death tells Donnie that every living creature on this earth dies alone.

Other teacher tells Donnie about time travel, and Donnie realizes that Grandma Death once wrote a book about time travel. The hallucinations Donnie experiences are found in the book.

Frank also tells Donnie to set this motivational speaker's house on fire. When the firefighters/police come to clean it up, they find that he was running a kiddie porn ring. He gets arrested.

The night the world is supposed to end, Donnie's sister Elizabeth has a party because their parents are gone. The youngest sister (Samantha Darko / Daveigh Chase) has gone off to a Sparkle Motion dance competition with her mommy.

Gretchen shows up to the party because her step father has come back, her mother is missing, and she's afraid something horrible has happened. Donnie leaves with Gretchen, they run into some thugs. Thugs beat Donnie up, throw Gretchen into street. Some kid accidentally runs over her. The kid gets out, and he's wearing the same BUNNY SUIT. He is Frank, Eizabeth's boyfriend. Donnie shoots Frank.

The plane that Samantha and her mom are traveling on is crashing, and the engine falls to the ground.

Donnie takes a portal back in time, and reaches his room. He laughs to himself until the jet engine from his mother's plane falls on him. He dies.

Gretchen is going by a house on fire. Sees Donnie's entire family outside, crying. Asks a neighbor about it - they tell her Donnie is dead. They ask if she knew him. She says, "No."
Donnie's mother waves to her, and she waves back.

You see a montage of people being sad. But at least kiddie porn guy hasn't been arrested, and Frank has not died. Yay?

Before Donnie dies, he writes Grandma Death a letter, so that she will have mail when she opens her mailbox

And, at the beginning of the movie, Donnie and his mom have a fight about taking his pills. Donnie calls her a bitch.
Later in the movie, he apologizes and tells her he just wants to be normal. And she says she loves him and couldn't ask for a better son.

So when Donnie goes back in time and dies, he and his mom never make up, so the last thing Donnie's mom ever heard him say was him calling her a bitch