In 1970s South Boston, FBI agent John Connolly (Joel Edgerton) persuades Irish mobster James 'Whitey' Bulger (Johnny Depp) to collaborate with the FBI in order to eliminate their common enemy: the Italian mob. Black Mass tells the story of this unholy alliance, which spiraled out of control, allowing Bulger to evade law enment while escalating his power to become one of the most notorious gangsters in U.S. history.