Although Black Mass centers on Boston-based mobster James "Whitey" Bulger (Johnny Depp), the biographical crime drama has viewers wondering what, exactly, happened to FBI agent John Connolly (Joel Edgerton) in the wake of his partnership with Bulger. Based on the book Black Mass: The True Story of an Unholy Alliance Between the FBI and the Irish Mob, Depp's controversial crime movie delves into what unfolded in Boston during the mid-1970s. Bulger, who helmed the Irish Mob's Winter Hill Gang, was the most powerful organized crime leader in South Boston, even though his family's roots caught Connolly's attention.

Growing up in the tight-knit South Boston community, the FBI agent had been a friend of both Whitey and his brother, another member of Black Mass' cast of characters. In a strange twist of fate, Whitey becomes an informant for Connolly, who hopes to infiltrate a rival Italian crime syndicate the Angiulo Brothers.