One night in London, Jimmy, a frightened 15-year-old, dashes through a shabby housing estate begging for help as passersby ignore him. He slips into the condemned Tower Block 31, the only floor still inhabited while tenants wait to be rehoused. He bangs on doors just as two masked men, who have been chasing him, close in. The only person who comes to his aid is office worker Becky, but she is beaten, and Jimmy is dragged away. The next morning, Devlin, a detective constable, conducts door-to-door inquiries about the previous night’s murder, after the boy’s body is found. Nobody will speak to him; not even Becky, who claims her injuries are the result of a mugging.
Three months later, Becky wakes in her flat beside her colleague Ryan, with whom she has shared a one-night stand after an office party. As they share breakfast, Ryan is suddenly shot in the head through the window. Becky flees into the hallway, along with the other residents, as every flat becomes a target: young couple Amy and Jeff; Carol and her teenage computer game-addicted son Daniel; a single mother Jenny; a middle-aged former soldier Neville and his wife Violet; small-time drug dealers Gary and Mark; a lonely alcoholic Paul; and the violent tenant Kurtis, who extorts protection money from his neighbors. The casualties mount: Amy and Mark are wounded, and Carol’s husband Brian and Jenny’s young children are killed by the sniper. Amy succumbs to her injuries despite Violet’s first-aid efforts. The phones and internet fail, and the walls are plastered with posters bearing three emoticons. Daniel, having learned from computer games, notes that the sniper is calculating and using a high-powered rifle.
The fire escape becomes exposed to gunfire, forcing the group to pry open the lift door, only to trigger a shotgun boobytrap that kills Jeff. Becky, Kurtis, and Paul descend the lift shaft, but find the ground-floor exit blocked by a skip and must retreat to the top floor. Jenny suggests the emoticons symbolize the three wise monkeys, positing that the sniper targets those who refused to help the authorities after Jimmy’s death. She then returns to her flat and is shot dead along with her children.
The largest poster is placed on the door of flat occupied by Gary and Mark. Kurtis forces them to admit they were the two masked men who chased and assaulted Jimmy, though they claim no intention to kill him. Nevertheless, Kurtis drags them into the flat, where the sniper kills them.
Believing the killers are now dead, Carol sprints down the fire escape, followed by Daniel. The sniper lets them reach the ground, then executes them, revealing his plan to punish everyone for not aiding the police. The survivors have now endured two days of entrapment. They devise a plan to lash the building’s fire hoses together and send one person down from the roof on the far side where the sniper can’t see. Paul volunteers to descend. Violet opens the roof door but is felled by another shotgun trap. Becky, Kurtis, and Paul reach the roof and secure the hoses, and Paul begins to lower himself. The sniper shoots the hose’s top, causing Paul to fall to his death. Becky and Kurtis scramble back inside.
In a final act of desperation, they set a fire on the top floor to draw attention and sprint down the lift shaft. Kurtis breaks his leg in the fall. Property developer Kevin and his assistant Eddie arrive, notice the flames, and are shot before they can summon help. The gunman enters the building, tossing in a smoke grenade. Becky escapes to find a nail gun in Kevin’s pickup truck. With that tool, the three survivors manage to restrain the gunman and remove his mask, only to discover it is DC Devlin, a man driven to the edge by a relentless lack of support in his cases. After a struggle, Becky disables him with the nail gun, and the survivors emerge to the sound of approaching sirens.
