Harry Brown (Michael Caine), an ex-marine and Northern Island pensioner, lives in a much maligned apartment block in South London that is overrun by violent youths. His wife is dying in the hospital and his best mate, Leonard (David Bradley) - with whom he plays chess with in the local pub, is intensely frightened by the goings on in their neighbourhood, and feels forced to carry around an old bayonet for protection. Harry visits his wife everyday, but always avoids a quicker route to the hospital via an underpass, as it is a favourite hang out of the aggressive troublemakers. This proves costly as one night he receives a phone call alerting him to his wife’s worsening health. Avoiding the shortcut again, he arrives too late to find she has died. On top of this, a few days later, Leonard is accosted after a gang of youths vandalise his home and is murdered by his own weapon. This proves too much for Harry and his disheartening faith in a weak police effort to do something leads him to take methods into his own hands.