In South Boston, where the state police force is waging war on organized crime, young undercover cop Billy Costigan is assigned to infiltrate the mob syndicate run by gangland chief Costello. While Billy is quickly gaining Costello's confidence, Colin Sullivan, a hardened young criminal who has infiltrated the police department as an informer for the syndicate, is rising to a position of power in the Special Investigation Unit. Each man becomes deeply consumed by his double life, gathering information about the plans and counter-plans of the operations he has penetrated. But when it becomes clear to both the gangsters and the police that there's 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 himself.