IN 1970s South Boston, Federal Bureau of Investigation (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.

Directed by Scott Cooper, “Black Mass” tells the story of this unholy alliance, which spiraled out of control, allowing Whitey to evade law enforcement while escalating his power to become the most feared crime lord in Boston and one of the most dangerous gangsters in US history.

The film is based on the 2001 book, “Black Mass: The True Story of an Unholy Alliance Between the FBI and the Irish Mob,” by Dick Lehr and Gerard O’Neill. The explosive revelation that Whitey Bulger had been an informant for the FBI made headlines in The Boston Globe in 1988 and, over the next decade, the details of the corruption spilled out.

“Black Mass” explores how a deal between ruthless gangster Whitey and FBI agent Connolly enabled Whitey to expand his criminal empire with complete impunity, as Connolly—blinded by his own ambition—shielded him from investigation, ignoring the rising body count.

For more than a decade—until his capture in 2011, Whitey was Boston’s most infamous crime lord; he was hunted by the FBI, surpassed only by Osama Bin Laden at the top of the bureau’s Most Wanted List. But the ironic fact is that Whitey might never have risen to the level of power he achieved were it not for the aid and abetment of the FBI.

“This story interested me because of the bond between these two brothers, who could not have been more different, and Connolly, who understood the power of the Bulger clan and had always revered them,” Cooper said. “Connolly ultimately allowed Bulger to run amok in the city because he’d wanted to be in Whitey’s good graces ever since Whitey rescued him in a playground fight when they were kids.”

“Many people grew up kind of idolizing him [Whitey],” Depp said. “Many wanted to be him because he did things his own way and, for the most part, he won. But he was also a very charismatic man. He had this draw that made people want to get close to him. They wanted to understand him. They wanted to know him.”

“Black Mass” is rated R-16 and is now showing nationwide.