The true story of Whitey Bulger, the brother of a state senator and the most infamous violent criminal in the history of South Boston, who became an FBI informant to take down a Mafia family invading his turf.

In late-1970s Boston, the FBI are looking to break the Mafia's hold on the north of the city. Agent John Connolly (Joel Edgerton) is on the case and willing to go to any lengths to achieve his aims. A South Boston native, he decides to "ally" the FBI with South Boston crime kingpin James "Whitey" Bulger (Johnny Depp), as his main enemies are the Mafia.

Approaching Bulger via his brother Billy Bulger (Benedict Cumberbatch), a Massachusetts State Senator, Connolly is successful in gaining Bulger as an informant and ally against the Mafia. However, this alliance only emboldens Bulger in his quest for power and wealth, as he is able to operate with impunity and utter ruthlessness. Not realizing he is being duped, Connolly finds himself backing Bulger rather than law, order, and justice.