In the gritty world of South Boston, the Massachusetts State Police are waging a war against organized crime. At the heart of this struggle is an attempt to infiltrate the operations of Frank Costello, the area's infamous Irish-American mob boss. The police plant a young undercover officer, Billy Costigan, within Costello's organization. Costigan is quickly indoctrinated into the world of violence and corruption, all the while reporting back to his superiors. Despite his commitment, the constant threat of exposure pushes him to the limit, psychologically and emotionally, as he delves deeper into his criminal persona to maintain his cover.

Parallel to Costigan's journey, Colin Sullivan, a member of the Special Investigations Unit, rises through the ranks of the state police. Unbeknownst to the department, Sullivan is actually a mole, groomed from a young age by Costello himself to act as his informant within the police force. As both men become deeply embedded in their respective roles of spy and informant, their lives become dangerously intertwined. A tense cat-and-mouse game ensues where each side of the law is desperate to identify the traitor in their midst before their own operative is compromised. With the stakes at their peak, Costigan and Sullivan find themselves locked in a high-stakes race against time, where only one can survive the violent collision of their dual identities.