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.