The movie begins with Detective Sergeant (DS) Tom Brant (Jason Statham), a hot headed police officer from a turbulent South East London police station, beating up three youngsters, who are trying to steal a car, with a hurley.
Brant is approached by PC Elizabeth Falls (Zawe Ashton) asking to help her in passing her Sergeant's exam. Brant advises her to give it some time as she has come out of rehab recently. Falls had previously worked undercover for the drugs squad and became addicted. Brant is also warned by his superior (Nicky Henson) to control his actions. It transpires during a session between Brant and a police psychologist who mentions incidents involving bugging the Superintendent's office, beating a person unconscious in a billiards hall and the earlier assault on the three youths.
Brant goes to the funeral of the Chief Inspector Roberts's (Mark Rylance) wife. The same night PC Sandra Bates (Elly Fairman) is shot dead by an unknown assailant.
Sgt Porter Nash (Paddy Considine) is transferred from West London Branch as Acting Inspector, in place of Roberts who is on extended leave. A boy approaches PC Falls in her home and asks for help as he thinks he killed someone during a gang attack. Falls asks Brant for a favour and he gives her the name of Detective Inspector (DI) Craig Stokes (Luke Evans). The Shooter, Barry Weiss (Aidan Gillen), calls reporter Harold Dunlop (David Morrissey) and tells him that he is the killer. He asks Dunlop to pick between seven and eight and when Dunlop picks eight, Weiss tells him that's the number of cops he will kill. Weiss walks down the street and shoots PC Theo Nelson (Joseph Dempsie), who is sitting in his car.
The South East London station is thrown into chaos due to the shootings. Brant meets with an informant Radnor (Ned Dennehy) who points him in the direction of Weiss, telling him that Weiss had previously boasted about setting a police dog on fire "for practice". Brant meets Nash and tells him about his personal problem of recurring blackouts. Nash tells him that Brant might be experiencing burn out based on his personal experience, as well as confiding that he once attacked a paedophile in his house as there was not sufficient evidence to convict him. Brant tells him about the lead and they both decide to visit Weiss. Meanwhile Chief Inspector Roberts rejoins his duty. At Weiss's apartment Brant recognises him as the guy whom he beat in the billiards hall.
Weiss panics after the visit and flees his apartment. Unknown to both parties Radnor is spying on Weiss and, on seeing Weiss leave, Radnor sifts through his rubbish and finds a parking receipt. Weiss follows Roberts to his apartment and after a struggle, he kills him by pulverising his head with a hammer. Realising his trail of forensic evidence he steals a police uniform and Roberts's badge and burns the flat. He then calls Dunlop and tells him about the murder and that he is now calling himself Blitz. Nash informs Brant about the incident.
Radnor goes to the parking garage and finds Weiss's car. He opens it and finds trophies of his kills. He calls Dunlop and asks for £50,000 in exchange for revealing the killer's identity. Falls goes to meet DI Stokes and asks him to bury the name of the boy and in return Stokes asks her out for a drink. Dunlop meets Radnor and they go the parking garage where Radnor shows him the car and the evidence in it. However Weiss sees this and quickly removes the evidence from the car. He follows them to a bar and kills Radnor in the men's toilet before Radnor can reveal his identity to Dunlop.
Dunlop tells Brant and Nash about his meeting with Radnor and the location of the parked car. When they arrive, however, the car is empty and the building's CCTV is faulty, giving them no evidence on Weiss. Falls and Stokes meet for a drink, as Weiss watches them from across the street. In the station Brant finds out the connection between the dead police officers and Weiss. Weiss is attacking all the cops who arrested him and Falls is next in line. Stokes drops Falls at her home and says he will call her tomorrow, which Falls interprets as rejection and angrily leaves the car.
Weiss attacks Falls but the boy who Falls was trying to protect interrupts and is killed instead. Brant and Nash visit Weiss's apartment and, on finding it abandoned, decide to release his photo to the media. A taxi driver identifies him and leads the police to his location. He briefly escapes but is pursued by Brant and caught in a train yard near Paddington. Falls, traumatised by the attack, has a relapse and steals drugs from a dealer.
At the station Brant, Nash and the Chief Inspector discuss that Weiss may walk due to the lack of evidence. Later Brant talks to Nash about the paedophile incident but Nash declines to talk any further. Brant taunts Weiss about his name 'Blitz' and tells him he is nothing, but is unable to provoke him into admitting to the killings. Meanwhile Stokes finds out from his informant that Falls is stealing drugs from dealers and contacts Brant. They reach her house and find her intoxicated, then throw away the drugs and help her regain her senses.
Weiss is released but placed under surveillance. He returns to his flat and changes into the police uniform stolen from Roberts's flat, then dodges the surveillance and drives to Roberts's funeral. There, he finds Brant and at that time Brant receives a text message and leaves. Weiss follows him to the parking garage he had previously left his evidence in.
After following him up a flight of stairs, Weiss aims his pistol at Brant and tells him to face him. But when he turns around, it is Nash and Brant attacks Weiss from behind with a crowbar. Nash picks up the pistol Weiss dropped, the same gun used in the previous killings, and hands it to Brant. Weiss quickly realises Brant's intentions and warns him how his death would be treated by the media. Brant informs him that there was no evidence, that Weiss was cleared and is no longer under police investigation, that the real 'Blitz' was partial to people in uniform and shooting them with the same pistol he is holding. Brant then shoots him in the head.
Brant and Nash leave the rooftop with Brant joking that the case might remain unsolved. The film ends with Brant releasing two dogs on Harold Dunlop.