In the rough crime-ridden neighbourhood of Lambeth in South London, nurse Samantha 'Sam' Berrins is mugged by a gang of Black British teenagers: Pest, Dennis, Jerome, Biggz and their leader Moses. However, something crashes from the sky into a nearby parked car. Sam uses the confusion to run away. Moses tells the gang to "allow it"; instead he wants to see if there is anything to steal from the car. When he approaches it he is attacked and wounded on his face by a creature, which he kills with the help of the others. Realising that it is something rare, they hope to gain a profit from it and go to drug dealer Ron (Nick Frost) for advice. Ron lives on the top floor in the same 25-story high-rise apartment block where the gang lives with their parents and legal guardians.
Ron and Brewis, a young zoologist who had come to buy marijuana, agree that it is a strange creature. Moses asks the owner of the flat, and Ron's boss, gangster Hi-Hatz, if they can keep it in the "weed room", as it is the safest place in the block. He agrees, if Moses, who sells weed for him, starts selling cocaine too, which Moses accepts.
While the gang looks out at the city, they see more creatures come down from space in small ships resembling shooting stars, and the boys each go to their home to collect machetes, softball bats, cricket bats, swords, switchblades and firework rockets. Their parents are not told anything. Dennis is told by his father to take their dog for a walk. Two young kids, calling themselves Probs and Mayhem, also go to the crash site, eager to prove how tough they are.
However, the gang sees that these new aliens are bigger with black fur, huge claws, and light-emitting teeth. They kill Dennis' dog, and the boys flee on foot and on their motorbikes. During the chase, the police catch Moses while the others hide and watch. Sam, in the police van, confirms that Moses was the mugger, but two aliens kill the policemen, and attack the van. The others chase the aliens away with a large firework rocket, and Dennis approaches and lets Moses out of the holding cell in the back; Sam enters it to protect herself from both the aliens and the gang. The three are attacked again but manage to drive off in the van. However, they crash into Hi-Hatz' car in an underground garage. Hatz is enraged. As Sam runs off, the other gang members catch up to Moses and Dennis, just as Hi-Hatz pulls a gun out. Hatz does not believe their story about aliens, until they spot one. Hatz forces his henchman to check it out, but the henchman is then killed by the alien, which Hatz kills with his pistol, as the gang run off.
Again chased by aliens, they attempt to get back to the block, but Biggz gets left behind and hides in a garbage container, and an alien bites into Pest's leg. They see Sam going into her flat, and force their way in. She tells them to leave, but they assure her that they are on the same side now, united against the aliens. Sam treats Pest's leg. An alien bursts through her front door, but Moses kills it. Sam is impressed, and decides it is safer to stay with the gang. The gang agrees after Pest convinces the others that she, being a nurse, can still be needed.
Hearing dozens of more aliens coming by climbing up the building, they all move to the apartment of some neighbourhood girls named Tia, Dionne, Dimples, and Gloria. They are met with skepticism, but two aliens attack through a window, killing Dennis. One alien is killed by Tia and the girls with a lamp and a skating boots blade, but the second alien is about to kill Moses when Sam comes to his rescue, killing it with a kitchen knife.
The girls, recognizing that the creatures only want the boys, kick them out and flee to the street. The gang and Sam decide to go to Ron's safe room on the top floor and hide out there, but are shot at by Hi-Hatz and his two goons. However, they escape. Hi-Hatz and his two goons, being attacked by an alien, try to hide in an elevator, but the goons are killed. Hi-Hatz manages to kill the alien and continues to search for Moses.
The gang, joined by Brewis, make their way to Ron's apartment but Jerome is killed by the aliens along the way. They reach Ron's apartment only to find Hi-Hatz inside waiting for them. However, aliens who have climbed the outside of the building attack through the window and kill Hi-Hatz. The survivors; Moses, Pest, Brewis and Sam reach the safety of the weed room.
Meanwhile, Biggz, still trapped in the garbage bin on the street by one persistent alien, phones for help, but nobody can help because the police, investigating the death of the two officers, have cordoned off the streets around the apartment block. However, Probs and Mayhem use their super soaker water guns (filled with petrol) to set fire to the alien.
Up in the weed room, Brewis sees liquid on Moses' jacket under the UV light and concludes that the first, small alien was a female, who left a pheromone on Moses, and that the black bigger males are attracted to the smell. They also realize that the aliens are blind and use their scene of hearing and smell to look for victims. After they check that Sam does not have any of the pheromone on her, she goes to Moses' empty apartment, where she opens the gas valves to her stove, then leaves the tower block safely. Moses and Sam have come to respect each other, and Moses tells Pest to return Sam's robbed ring to her, which Pest reluctantly does.
Moses then straps the dead female alien to his back and runs out of the weed room to his apartment, with all the aliens in tow. Moses throws the dead female alien into his kitchen, where all the males follow, ignites the gas with his cigarette lighter, and jumps out of his flat, just escaping the explosion which apparently kills all the aliens. Sam, the girls, Biggz, Probs, Mayhem and other people watch as Moses manages to grab onto a British flag hanging on the balcony and survives.
Pest, Brewis, Ron and Moses are arrested by the heavily armed Metropolitan Police that show up, but Sam tells the police that they actually saved her life. In the back of the police van, Moses and Pest hear the crowds chanting for Moses, and Moses smiles, knowing that this time he has done something heroic.