Harry Brown is an elderly pensioner who was once a decorated soldier in the Royal Marines, and a veteran of Northern Ireland. He lives on a London council estate ruled by violent gangs and spends most of his time playing chess with his friend, Len Attwell, at a local pub owned by Sid Rourke. When the hospital phones to tell him that his wife, Kath, is dying, Harry is too late because he is scared to take a shortcut through a narrow underpass, where a gang holds court. His wife is laid to rest next to the grave of their thirteen-year-old daughter, Rachel, who died in 1973.
Len confides to Harry that he is being terrorized by youths and shows him an old bayonet he now carries to defend himself; with the police unable to help, he plans to confront his harassers himself. The next day, Harry is visited by Detective Inspector Alice Frampton and Detective Sergeant Terry Hicock, who tell him that Len has been murdered. The police arrest Noel Winters, the leader of a drug-dealing gang, along with members Carl, Dean and Marky, but are released due to lack of evidence. Harry gets drunk after Len's funeral, and is then held at knife-point by Dean, who intends to rob him; Harry kills Dean with his own knife during a brief struggle. Frampton visits Harry in the morning to inform him that because Len was killed with his own bayonet, any charges could be reduced to manslaughter on the basis of self-defense, leaving Harry feeling angry and betrayed.
Harry follows a drug dealer, Kenny, to a den where he negotiates to buy a pistol. Inside, Harry finds Kenny and his associate, Stretch, growing cannabis and sexually abusing an overdosing girl to make pornographic films. Harry kills the dealers before burning down their den, fleeing with the girl and a bag of firearms in a stolen Land Rover. He leaves the girl outside a hospital, and later follows Marky to find him being sexually abused by Troy Martindale, a heroin smuggler that supplies drugs to the gang. Harry shoots and kills Martindale and captures Marky, whom he tortures into revealing some mobile phone camera footage of Len's murder, proving the gang did not kill Len in self-defense. Harry then uses Marky to bait Noel and Carl into a gunfight in the underpass; Carl and Marky are killed, and Noel flees the underpass with Harry in pursuit, only for Harry to collapse from an emphysema attack. Harry is found and taken to hospital.
Frampton has deduced that Harry is behind all the recent shootings, but her boss â Superintendent Childs â is instead convinced that they are all part of an escalating gang war; he thus orders a major arrest operation on the estate, which results in a riot. Harry discharges himself from the hospital to pursue Noel. Driving onto the estate to stop him, Frampton and Hicock are involved in a car crash in which Hicock is severely injured. Harry rescues them and takes them to Sid's pub, where Frampton warns Harry that Sid is actually Noel's uncle. Harry discovers that Sid has been hiding Noel, but his guard drops due to his emphysema, allowing Sid to disarm him and reveal himself to be the gang's real leader. Frampton tries to call for backup, but is stopped by Noel, who begins to strangle her after Sid suffocates Hicock to death. Though weakened, Harry draws a concealed revolver and kills Noel; Sid prepares to kill Harry in retaliation, only to be shot dead by police snipers, who have responded to Frampton's call.
At a press conference held after the riot, Superintendent Childs announces that Frampton and the late Hicock are to be awarded posthumously, but denies any evidence of vigilante involvement in the case. The film ends with Harry â seemingly recovered â walking towards the underpass, which is now quiet and gang-free.
Harry discharges himself from what?
hospital