South Boston cop Billy Costigan (Leonardo DiCaprio) goes under cover to infiltrate the organization of gangland chief Frank Costello (Jack Nicholson).
As Billy gains the mobster's trust, a career criminal named Colin Sullivan (Matt Damon) infiltrates the police department and reports on its activities to his syndicate bosses.
When both organizations learn they have a mole in their midst, Billy and Colin must figure out each other's identities to save their own lives.