Donnie Darko suffers from paranoid delusions which manifest when he sleepwalks. Regularly medicated to keep them under control, one night a giant rabbit tells him that the world will end in 28 days, 6 hours, 42 minutes, and 12 seconds. When he returns home, a jet engine has crashed through the roof of his house, starting a chain of events that count down to October 31st, 1988.

Gretchen turns up at Donnie's party in tears after her mom has skipped town and left her behind. They go to the bedroom together. Later Donnie notices on the fridge a note left by Frank, saying he left to get beer.
  He tries to avoid the light-tunnel destiny paths so they race off to find Grandma Death and find answers to stop the end of the world. He steps down through the cellar door but finds the two bad kids already inside stealing her stuff.
  Outside there is a fight and Gretchen is thrown to the ground. Grandma Death steps out into the middle of the road, and Frank's car swerves to avoid her -- it mounts the kerb and hits Gretchen.
  The bad kids run off. Frank gets out the car wearing his bunny outfit. Donnie picks up the gun and shoots Frank in the eye.
  Donnie carries Gretchen's body home, then steals his sister's keys and drives them both to the top of the hill overlooking town from the first scene and waits. A huge tornado wormhole is forming over his house.  Meanwhile, on a flight returning home, Rose and Samantha are hit with turbulence - and one of the jet engines breaks loose and falls through the time funnel.

Suddenly, we are back in time. Donnie is lying in bed, aware of what will happen, and laughs as the jet engine crashes through his bedroom ceiling.
  It cuts to the rendition of 'Mad World' by Gary Jules, as it cycles through the main characters waking up that morning - Swayze burdened with grief, Frank with the mask, the psychiatrist etc.

In the morning, the Darko family are waiting outside the family home - all distraught except Rose who is smoking, contemplating. Gretchen rides towards the incident and asks a local kid what happened. She waves at Rose, who returns the motion.

Look carefully for the cameo by Seth Rogen!