An undercover cop and a mole in the police attempt to identify each other while infiltrating an Irish gang in South Boston.In this crime-action tour de force, the South Boston state police force is waging war on Irish-American organized crime. Young undercover cop Billy Costigan is assigned to infiltrate the mob syndicate run by gangland chief Frank Costello. While Billy quickly gains Costello's confidence, Colin Sullivan, a hardened young criminal who has infiltrated the state police as an informer for the syndicate is rising to a position of power in the Special Investigation Unit. Each man ... More from IMDb