In 1975, a low-level Boston gangster named Whitey Bulger makes the deal of a lifetime: he becomes an informant with the FBI.
(Yeah, we know. You're hooked on this story already.)
Brought on by agent John Connolly, a former childhood friend, and given protection by agent John Morris, Bulger immediately starts feeding the FBI inside intel on his foes, usually as a roundabout way to benefit himself. Clever boy.
Things get corrupt fast. It starts when Bulger is accused of shaking down a restaurant owner, with Morris and Connolly diverting attention from their favorite informant, and just continues from there. Race-fixing scandal with Bulger clearly involved? No biggie. Bulger's name being mentioned recorded on FBI wiretaps? Not the way Connolly hears it.
As the years pass, Bulger becomes the most well-known—and powerful—gangster in all of Boston. This is, of course, due to the support of the FBI, have protected Bulger from prosecution time and time again by the time the 1980s roll around. It also helps that Whitey's little bro, Billy, is a prominent politician, having served as the president of the Massachusetts state senate for years.
Meanwhile, Whitey's deal with the FBI has gotten downright dirty. Connolly gives Bulger a heads-up when a low-level G named Brian Halloran is going to rat him out, so Bulger straight up murders him. The DEA and Boston PD have also honed in on Bulger by this point, but Connolly always manages to disrupt their investigations.
This corrupt crew—Connolly, Morris, Bulger, and another informant named Steve Flemmi—start holding meetings at Morris' home, where they discuss their dirty deal and how to protect it from nosy agents. It's at one of these fateful meetings that Morris makes Flemmi and Bulger an important promise: they'll be free from prosecution as long as they don't kill anyone.
But kill they do, continuing to build up their criminal empires until one fateful day in 1995 when the FBI, led by prosecutor Fred Wyshak, finally takes its shot at Bulger and Flemmi. They're both indicted as part of an investigation into the Mafia and, though Bulger manages to skip town (thanks to a tip from Connolly), Flemmi is caught soon after and lands behind bars.
As a legal defense, the mobsters' lawyer Anthony Cardinale argues that Bulger's status as an informant—which has been rumored for years—should cause the case to be thrown out. In order to decide whether this argument is valid, the judge must make public all available information on the FBI's deal with Bulger.
This'll go swell.
So the judge orders the information released, and it's like a bombshell goes off in Boston. John Morris is given immunity in exchange for his testimony against Bulger, while Connolly refuses to do anything except plead the Fifth. When everything's said and done, however, Flemmi is sentenced to prison, presumably for the rest of his life, as is Connolly, who's officially convicted as an accessory to murder in 2008.
As for Bulger, the gangster stays on the run until 2011, when he's discovered in Santa Monica, California, living under an assumed name with his long-term girlfriend. They were discovered after a neighbor recognized them on the FBI's Most Wanted list. Although it's taken about sixteen years for the feds to catch up with Whitey, this brutal gangster is about to meet a little thing called justice.