In a gripping voiceover, Irish-American mobster Frank Costello articulates his desire to shape his environment rather than be molded by it. The film begins with grainy historical footage that illustrates the violent riots that unfolded in Boston during previous generations. Costello, who wears his Irish heritage with pride and harbors unapologetic racist sentiments, expresses contempt toward African Americans for their perceived complacency, insisting that "you have to do whatever possible to take what you want."
One tense scene unfolds in a small convenience store, where Costello aggressively collects protection money from the visibly anxious shop owner. His threatening demeanor escalates as he queries the shop owner's teenage daughter, leaving her both affronted and oddly drawn to his terrifying aura. Watching this interaction is a young boy named Colin Sullivan (Conor Donovan), who becomes a pivotal figure in Costello's world. After a curt warning to ensure he brings more money next time, Costello instructs the shop owner to give the boy two bags of groceries and slyly intimates that when he grows up, he can earn more from Costello.
As time passes, Sullivan grows up under Costello's influence, ultimately landing a job as a police officer who will keep tabs on law enforcement activities. He is welcomed by his colleagues, including Captain Queenan and Staff Sergeant Dignam (Wahlberg), into the Special Investigations Unit focused on organized crime—a unit that is ironically targeting Costello.
Meanwhile, Billy Costigan, a character with deep familial ties to organized crime, is also at the police academy. He faces harassment from Queenan and Dignam, who suggest he’s too intelligent for the job but eventually coerce him into going undercover. They propose a risky plan: Costigan must serve time in jail for a fabricated assault charge to infiltrate Costello's mafia organization, which only Queenan and Dignam will know about.
Upon his release, Costigan tries to establish credibility with the mob, meeting with Mr. French (Ray Winstone) at a bar, where a confrontation spirals into violence. Costigan then encounters Italian mobsters shaking down store owners, showcasing his readiness to fight for respect in the criminal underworld. Despite the risks, Costello recognizes Costigan’s potential and offers him protection, further consolidating Costigan's role within the organization.
As both Costigan and Sullivan rise through their respective ranks, they remain unaware of each other's identities as moles. Sullivan begins a relationship with psychiatrist Madolyn Madden (Vera Farmiga), who also sees Costigan professionally, creating a complex triangle of attraction and betrayal. Tensions escalate as Sullivan, who is tasked with finding the infiltrator in his unit, wrestles with his dual allegiance while Costigan gathers vital information about Costello's operations.
In a series of intense confrontations, Costigan discovers that Costello is, in fact, an FBI informant. This revelation leads to a climactic series of events where characters face betrayal and violence. In a pivotal moment, Sullivan is seen as a hero for taking out Costello, but the underlying duplicity continues to unfold.
The story reaches its tragic conclusion when Costigan, seeking vindication and a return to normalcy, confronts Sullivan, leading to a deadly encounter marked by dramatic twists. With haunting irony, Sullivan’s survival tactics ultimately lead him to a downfall orchestrated by his own machinations.
As the film closes, a poignant symbol of betrayal—a rat—scurries across the backdrop of a glimmering Boston skyline, encapsulating the dark themes of loyalty, treachery, and identity entwined throughout this thrilling narrative.
