The South Boston state police force is fighting Irish-American organized crime in this crime-fighting masterclass. Billy Costigan, a young undercover detective, is tasked to infiltrate the crime organization led by gangland boss Frank Costello. While Billy swiftly gains Costello's trust, at the Special Investigation Unit, Colin Sullivan, a tough young criminal who has joined the state police as an informer for the syndicate, is rising to a position of prominence.
Each individual is preoccupied by his double life, accumulating knowledge about the operations' plans and counter-plans. However, as it becomes evident to both the mob and the police that there is a mole among them, Billy and Colin find themselves in jeopardy of being discovered and exposed to the enemy - and each must race to discover the identity of the other guy in time to rescue themselves. Is either of them willing to turn on the friends and comrades they've made throughout their long undercover stints?