 Billy Costigan, a young man from a crook family, wants to do redeem the Costigan family by joining the cops. However, the cops see him as the perfect mole because of his crook family, so they send him undercover to the Irish Mob. Meanwhile, Frank Costello, the mob boss, sends in Colin Sullivan to become a cop for the opposite purpose. Both get caught up in their double lives and become obsessed with finding the other mole, as does Costello and Queenan, the heads of each side.