Still affected by the tragedy of her older brother Donnie's death, Samantha (Daveigh Chase - the only surviving cast member from the original) is now 18 and on a roadtrip across the US to LA with her friend Corey when their car suffers engine failure and they end up in the small town of Conejo Springs while they wait on the repairs.  Samantha starts her sleepwalking again and suffers from Donnie-esque visions of the future - will she be able to stop the world ending on July 4th 1995?

Corey is drinking in the car with Randy and his friends and is nasty to her best mate who walks off alone. Randy tells her to get out and then speeds away, but is rammed by a mysterious driver-less black car and hits Samantha who dies.
  Corey takes it quite badly, understandably, and starts to see visions and timelines of her own. She manages to reverse time, is friendlier to Samantha while in the car so Samantha doesn't storm off, and dies in the car crash further down the road.

Samantha is now alone, Randy has lost his license so gets around town on a bike, and the meteor guy is getting a nasty rash.

It should have ended here but it drags on for about another hour. The pastor takes Sam to the cinema to explain that he looks at the blank screen for inspiration, and she sees her past performance in Sparkle Motion from the original. He tries to make a move, she leaves. Outside she doesn't tell his wife, who is acting strange and doesn't have the blue bracelet on her arm from before.
  Samantha continues sleep walking and finds a deserted mine on the day the world will end with the bodies of the kids inside. Outside the locked gate is the bracelet belonging to the pastor's wife.  She is a hero in town and everyone blames Iraq Jack the outsider.

That night the nerdy meteor guy takes her to the top of a hill to watch the 'fireworks' - all these cube shaped meteors heading down to earth. Affected by them in a "Smallville" style, he changes appearance a little and freaks her out. They have a fight and she is pushed onto a metal rabbit mask left behind by Iraq Jack and dies.

Iraq Jack now is the Donnie Darko figure and manages to reverse time. To stop everything that is about to happen, he stays on top of a tower waiting for the first meteorite to fall a few days earlier. He is killed instantly.
  Next day the nerd isn't allowed to take home the meteor as Jack had died, and the girls are both safe -- although the kids are still not rescued so its a depressing end to a terrible movie.

Apparently the writer studied "the philosophy of time travel" from the original so the movie sub-plot is all about alternative universes/realities, time-travelling artifacts that can make it through the wormhole (eg the jet engine/meteorite), the 'manipulated dead' and the 'living receiver'. Still doesn't make it any better - he should have picked out everything else that was good about the original and just used that instead.