In South Boston, the state police wants to end the reign of powerful mob boss Frank Costello. A young rookie, Billy Costigan, is assigned to infiltrate Costello's mob. Another young cop, Colin Sullivan, is among a handful of elite officers whose mission is to bring Costello down. But Colin is working for Costello, keeping the crime boss one step ahead of the police. Each man becomes consumed by his double life, but when it becomes clear to both the mob and the cops that they have moles in their midst, Billy and Colin must race to uncover the identity of the other man in time to save themselves.