Plot The Boston police department manages to place young cadet Billy Costigan (DiCaprio) deep undercover in the city’s Irish-American gangland, run by the violent Frank Costello (Nicholson). But gangster Colin Sullivan (Matt Damon) has also signed on to join the police force, planning to leak information back to Costello.Verdict Back to the streets and with a stellar cast, Martin Scorsese proves once again that he’s the master of urban storytelling — and of thrillingly violent filmmaking.