In England, retired Royal Marine Harry Brown lives a solitary life, dividing his time between the hospital where his wife Kath is terminally ill and playing chess with his only companion, Leonard Attwell, at Sid Rourke's pub. After Kath passes away, Len confides in Harry about being harassed by a local gang and mentions he has an old bayonet for protection. Harry advises Len to report the threats to the police. Tragically, Len is later beaten and stabbed to death.
Detectives Alice Frampton and Sergeant Terry Hicock arrive to investigate but bring bad news: they lack sufficient evidence to make arrests, aside from the bayonet, allowing the gang to potentially claim self-defense if taken to court. Realizing that the system won't provide justice, Harry decides to take action himself. Harry Brown (Caine) is an old ex-marine living in a rundown apartment in England.
The rising crime rate has made him so fearful that he avoids using the underpass near his home. Following his wife's death, Harry's closest friend is killed, which drives him to seek revenge against the criminals who have tormented him. He is a retired soldier whose only remaining connections are his wife Kath and friend Len. With Kath gone and Len murdered, Harry concludes that the authorities will not act on Len's death, prompting him to take matters into his own hands.