In South Boston during the 1970s, small-time criminal James "Whitey" Bulger forms an alliance with his childhood friend John Connolly, now an FBI agent. Bulger's brother, Billy, is a Massachusetts state senator with significant influence. Connolly persuades Bulger to become an FBI informant to help the agency eliminate a mutual rival, the Italian mob. This uneasy alliance allows Bulger to expand his criminal empire with relative impunity, believing the FBI's protection will shield him from law enforcement scrutiny. As Bulger's power grows, he becomes increasingly ruthless, engaging in drugs, extortion, and brutal violence.

Bulger's relationship with Connolly becomes dangerously personal and complex. Connolly, blinded by loyalty and ambition, compromises his principles and FBI duties. The dynamic among these three men—Bulger's unchecked rise in the underworld, Connolly's moral descent, and Billy's political career—sets the stage for an inevitable and dramatic reckoning. The FBI's internal conflicts and mounting suspicions begin to close in on Bulger's operations, threatening to expose the fragile alliance that once kept him untouchable.