Black Mass is a crime drama film based on the true story of notorious Boston gangster Whitey Bulger. Benedict Cumberbatch plays Billy Bulger, Whitey's brother and a prominent politician. The film depicts the intertwining worlds of organized crime and law enforcement, showcasing the corrupt alliances and betrayals that shaped Boston's criminal underworld. Cumberbatch's performance adds depth to the character of Billy, highlighting the complex dynamics within the Bulger family.