Robert McCall (Denzel Washington) is a man with a mysterious past, trying to live a quiet life.
A teenage girl he has casually befriended (Chloe Grace Moretz) is a prostitute who is being beaten and forced to work for a Russian mobster, and Robert cannot stand idly by.
He kills her pimps, which sets the Russian mob after him. He uses his skills to kill everyone who comes after him, including the head of the mob. He also secretly helps several of his coworkers in dealing with corrupt cops and an armed robber.
The girl is able to leave the mob's control and try to make it as a singer, and Robert decides to continue to use his skills to help people.