Shot in a gritty, documentary style with a cellphone camera, the film begins with a stark initiation into gang life, showcasing a boy from a South London council estate who is forced to take drugs and wield a gun. The narrative quickly escalates as he joyrides with two others on a motorbike, leading to a harrowing incident where they take the life of a mother walking with her child. Their reckless adventure ends tragically when the boy himself is struck by a lorry while fleeing the scene.
The plot centers on Harry Brown, portrayed by Michael Caine, an elderly ex-Royal Marine and veteran of the Northern Ireland conflict. Despite the escalating violence in his neighborhood, Harry attempts to maintain an air of indifference. Residing in a modest flat at the Heygate Estate in Walworth, London, he frequently visits his comatose wife in the hospital, deliberately avoiding a notorious underpass that serves as a hangout for a local gang. Much of his spare time is spent in a local pub, where he enjoys playing chess and drinking with his close friend, Leonard Attwell, played by David Bradley, while mutually navigating the untrustworthy ambiance provided by the pub's owner, Sid.
In a twist of fate, Harry receives a midnight phone call informing him that his wife is near death. Faced with the prospect of using the dangerous underpass to reach her, he succumbs to fear and arrives too late to say goodbye. Following her funeral, Leonard opens up to Harry about being bullied by local youths, showcasing an old bayonet he now carries for self-defense, lamenting that law enforcement refuses to listen to their pleas for help. Not long after, Leonard is horrified to find someone has pushed burning dog waste through his mail slot—a clear act of intimidation.
The situation escalates further when Harry is confronted by detectives Alice Frampton, played by Emily Mortimer, and Terry Hicock, who inform him of Leonard's murder at the hands of local gang members. Although some are apprehended, they are released due to lack of evidence. In the wake of Leonard's passing, Harry finds himself spiraling into despair and, after a chance encounter with one of Noel's gang members who attempts to rob him, he suddenly taps into his military training, confronting and destroying his assailant.
As Detective Frampton later informs Harry, the circumstances surrounding Leonard's death could lead to manslaughter charges for self-defense instead of murder. This revelation sparks a turning point for Harry, prompting him to embark on a dangerous path of vigilantism. He follows one of the gang members, Kenny, back to a decrepit house, where he extracts himself from a tense negotiation involving firearms. Inside, he discovers a disturbing scene of drug abuse and exploitation, which leads him to confront the perpetrators, resulting in their demise. Taking a significant amount of cash as he rescues an unconscious woman from their clutches, he also discreetly donates a portion to a local church.
Through persistent surveillance from Leonard's old apartment, Harry grapples with the street gang's dynamics. He captures a young recruit, Marky, and extracts vital footage proving the gang's role in Leonard's murder. Utilizing Marky as bait, Harry orchestrates a tension-filled showdown, facilitating further violence that leaves him gasping for breath in a climactic scene where he suffers an emphysema attack.
With the neighborhood descending into chaos due to a police crackdown ordered by Superintendent Childs, an intense riot erupts amidst the crackdown. Injured, Harry escapes the hospital, but Detective Frampton's attempts to halt him lead to a critical turning point when a vehicular crash occurs. In a desperate attempt to survive, Harry saves them from their attackers, only to unearth deeper connections between Sid and the gang leader, Noel.
The stakes rise sharply as Harry and the detectives confront treachery in the pub basement. After brutal confrontations and tragic deaths of his allies, Harry finally draws upon hidden resources to fight back, resulting in a deadly altercation that leaves him critically injured. As the dust settles, emergency services arrive, putting an end to a spree of violence that has shaken the community.
In the aftermath, a news conference celebrates the police's efforts in reducing crime, with rumors of a vigilante aiding their cause dismissed by the authorities. The film concludes on a contemplative note, showing a seemingly recovered Harry heading towards the now peaceful underpass, accompanied by a voiceover from Superintendent Childs praising the recent decline in crime as a community returns to a semblance of normalcy.
