In 1970s South Boston, FBI Agent John Connolly (Edgerton) persuades Irish mobster James "Whitey" Bulger (Depp) to collaborate with the FBI and eliminate a common enemy: the Italian mob. The drama tells the story of this unholy alliance, which spiraled out of control, allowing Whitey to evade law enforcement, consolidate power, and become one of the most ruthless and powerful gangsters in Boston history.