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 Costellos 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 becomes deeply consumed by their double lives, gathering information about the plans and counter-plans of the operations they have penetrated. But when it becomes clear to both the mob and the police that there is a mole in their midst, Billy and Colin are suddenly in danger of being caught and exposed to the enemy - and each must race to uncover the identity of the other man in time to save themselves. But is either willing to turn on their friends and comrades theyve made during their long stints undercover?
Two just-graduated officers from Massachusetts State Police Academy follow opposite sides of the law: Billy Costigan is assigned to work undercover with the Irish mobster Frank Costello to get evidence to arrest him. His true identity is known only by his superiors Dignam and Oliver Queenan. The protégé of Costello, Colin Sullivan, is promoted in the Massachusetts State Police and is the informer of Costello. Each police officer gives his best effort trying to disclose the identity of the other "rat."