A group of drug dealing gang members murder Harry Brown’s (Michael Caine) only friend, Leonard (David Bradley). Harry is a retired Marine and a widower so he decides to seek revenge and takes the law into his own hands. Harry is now a vigilante killer doling out his own brand of justice using his marine combat skills upon the unsuspecting gang members. Will Harry be able to finally settle the score with all the killers without being found out, or will the drug dealing gangsters be too much for him to handle in his pursuit through the streets of Britain?