In South Boston, the state police force are waging a war on Irish-American organized crime. Undercover cop Billy Costigan is assigned to infiltrate the mob syndicate, which is run by gangland chief Frank Costello. Billy quickly gains Costello's confidence. Colin Sullivan is a hardened young criminal who has infiltrated the police department as an informer for the syndicate. Colin is rising to a position of power within the Special Investigation Unit....

In South Boston, the state police force are waging a war on Irish-American organized crime. Undercover cop Billy Costigan is assigned to infiltrate the mob syndicate, which is run by gangland chief Frank Costello. Billy quickly gains Costello's confidence. Colin Sullivan is a hardened young criminal who has infiltrated the police department as an informer for the syndicate. Colin is rising to a position of power within the Special Investigation Unit....