We are at a luxurious cinema in Detroit. One of the employees, Paul (John Leguizamo) is carrying a small torch in his helmet, as if he were a miner. He tries to seduce the girl (Jordan Trovinion) at the cinema, when the lights go out. Everybody disappears, except for that man. He finds another survivor (P.J. Edwards), but he disappears as well, as soon as he is enclosed by the darkness. Everybody's clothes, glasses... are left behind, but there are no clue as to what happened to the bodies.
Rosemary (Thandie Newton) looks around at a deserted hospital. Nobody answers to her cries for help. She hears soft moans and goes to see who that might be. It's a man (Hugh Maguire) who was suffering some surgery. Now, he is alone, the anaesthetics have gone off. At that moment, the lights go off again and that man disappears. Rosemary stays because of the torch she's holding.
Luke (Hayden Christensen) wakes up. The lit candles from a night of passion with his wife Paige (Erin Nicole Brolley) saved his life. He can't find her and goes to work without noticing anything strange. He picks up a newspaper at the foyer hall of his building and leaves. He doesn't look up until he steps on somebody's glasses - and abandoned clothes. A plane falls off to the ground causing a commotion to which nobody answers. Luke comes back to his flat looking for his wife and daughter, but nobody is there. He goes to the editing room of Action News, and nobody is there either. Darkness closes in over the photograph of he and his family, his face the last one to disappear.
72 HOURS LATER (3 DAYS)
Outside the bar Sonny's, a man arrives, being unable to turn on his lighter. The shadows immediately claim him, because the bar owner didn't want to help him with his flashlight. Instead, he ravages on his remains after the shadows have claimed him. Luke arrives to that bar. James (Jacob Latimore) points a shotgun to him, but shoots to the wall. Luke and James start talking about the situation. James is waiting for his mother in a coffee bar which has its own power on. Luke says that the sun rises later each time, but sets down sooner, and that James' mother is not going to come back for him. Rosemary arrives, and tells how she was saved by the lighter she was using to light a cigarette. Back home, she finds that her baby is gone, and she is looking for it.
They hear somebody whining and moaning. Luke decides to take a torchlight and go see what happens. There is a man, wounded and trembling, at a lit bus stop shelter. Luke takes him to Sonny's while James and Rosemary are waiting inside, and James realizes that his mother is not coming back. Luke has to help him to his feet. falls down and his torchlight rolls away from them. The shadows lurk around him and try to catch them, but Rosemary opens the door for them. They are saved on the last second.
Paul (John Leguizamo) says that his light went off, and that he was hit by somebody from the darkness. He was taken somewhere else when suddenly his light went on again, so that he doesn't even know how he ended up at that bus shelter. They all argue about what caused the phenomenon: a black hole, a single occurrence, God... When the argument starts to get heated, with Rosemary getting frantic, the lights start to flicker, so that the shadows on the darkness close on in immediately. Paul advises to see to the generator in the basement, which solves the problem for the moment. The generator is not very modern, and it stubles down occasionally. Back there, Paul suggests that the universe is rebooting itself. They put him on the billiard table when he faints, and Rosemary applies ice to his wounds.
Luke is holding a pill, thinking about what he saw at the Action News studio. He checks the recordings of live problems, with news anchor people Stephen Clark and Carolyn Clifford-Taylor (themselves) speaking live right before the first blackout, talking about the blackouts somewhere else. He also sees his late wife Paige, the weather woman, who was taken by the lights.
Luke sees the last video message of a Chicago reporter, (Neal Huff), who says that his brother dead eight years ago called him from the darkness. The Chicago reporter says that the people on the shadows can trick you, and advises everybody to trust only the light they are holding.
Paul starts thinking that there must be a reason for them to be alive. Luke thinks it'd be better if they try to get out of Sonny's: they can't wait eternally inside the bar. He and Rosemary put some lights around their necks and bodies and leave in the middle of the night. Rosemary keeps wondering why, but Luke thinks it's all random. James turns on the juke box while Paul stays on top of the billiards table. James remembers how happy he used to be on Sonny's, with all the players, the patrons and the bar girl. He goes to the toilet and picks some water on the bowl. Paul thinks that James has gone off, disappearing taken by the shadows in the darkness.
When the lights are turned on again, James has disappeared. Outside, Rosemary keeps on reminiscing about her lost daughter. She and Luke check all the cars. A girl, Briana (Taylor Groothuis) appears out of nowhere with a flashlight. Luke tries to go after the girl, but he falls down. Rosemary turns on the lights of a car, which saves them both from the voices in the dark. They finally are able to turn on the lights when the lights at Sonny's fade away forever. Luke tells Rosemary that she has to keep her foot on the gas, or they will lose power.
Paul is seen running down a huge corridor lit by round lights. They go off one by one until the shadows take him. As soon as he is taken, lights go on and James reappears out of nowhere. He is in the toilet, picking up some water for Paul in the bowl, exactly what he was doing before disappearing. Paul's clothes are on top of the billiards table, from where they have never gone away. It looks that he died on the billiards table, not on the corridor.
Rosemary is looking for some fuel, pushing and moving around the things on the shelves, when she hears the crying of a baby. She goes to see whether it's her baby. The cradle is lit by a strong light which comes from somewhere over it. As soon as Rosemary is close to the cradle and looks in, the lights go off. Her void clothes fall to the ground and Luke sees it.
The shadows in the darkness seem to see James, who runs from them into a church. Inside, James sees a group of candles on the floor, with some blankets. He approaches the heap of clothes but nobody is there. He hears his mother softly calling him out as one clear voice among the whispers coming from the surrounding darkness. James doesn't want to go close to her. Luke crashes the car into the church, as he has seen James entering the place. where many candles are on. As the car lights flicker, Luke is caught in the middle, trying to run after James. Luke is taken. James stays with the candles, until they all vanish. James repeats over and over again: "I am James. I exist.", following one of the philosophical theories which Luke, Rosemary and Paul have defended: that people exist because they are conscious about their own physical existance.
It's morning. James wakes up surrounded by unlit candles. Briana says that the things he saw at the church - which he took for his mother's - are hers. Briana and James leave the church, and they see a horse eating some apples. Both James and Briana leave Detroit on the horse.
When night arrives, they have reached the outskirts, and they are on the road out.