Two brothers grow up in South Boston. One becomes a prominent politician, while the other turns into the state’s (excuse me, the commonwealth‘s) most dangerous gangster. Their childhood friend goes to work for the FBI. Why? We don’t find out from Black Mass, which makes an argument about human psychology that basically begins and ends with, “Boys from Southie stick together.”

Still, Scott Cooper’s new film is an absorbing chronicle of the unlikely, and ultimately tragic, association between James “Whitey” Bulger and the FBI—which relied on Bulger as an only occasionally enlightening informant for two decades starting in 1975, hoping to bring down the Italian mafia and seeing Bulger as the lesser of two evils.

The lesser of two evils is still, of course, an evil, and Black Mass centers on FBI agent John Connolly (Joel Edgerton): a childhood friend of Bulger’s who convinces the gangster to turn informant. “I don’t consider this snitching,” Bulger (Johnny Depp) tells Connolly. “I consider this business.” Very good business indeed—for Bulger. The partnership insulated him from FBI harassment while he rose to become Boston’s preeminent crime lord, even as his brother Billy (Benedict Cumberbatch) became President of the Massachusetts Senate.

At its heart, Black Mass is the story of Connolly’s corruption. Cooper, with screenwriters Jez Butterworth and Mark Mallouk, argues that Connolly was seduced by Bulger, ultimately becoming little more than a member of his gang who happened to have an in with the feds. The doughy Edgerton plays tough, though we see he’s feeling increasingly helpless as his “source” tightens the vice.

As with many crime dramas set in Boston, Black Mass is dark and somber. While gangsters in Scorcese films (including The Departed, which was loosely inspired by the Bulger/Connolly story) often seem larger than life, the characters in Black Mass look like they want nothing more than to be bodily absorbed by the soot-stained expanses that surround them—whether the claustrophobic streets of South Boston or the Brutalist edifice of City Hall. Composer Tom Holkenborg underscores the film’s events with elegiac strings that surge like an undertow, heightening the mood of thickness and despair.

The role of the seedier Bulger brother, known as “Jimmy” to his friends, is obviously a showcase for Depp, who doesn’t disappoint. The actor well-known for his collaborations with Tim Burton looks as cadaverous as Jack Skellington, his hair receding and his teeth yellowing (but his trademark aviator sunglasses always intact) as we follow him through the decades.

Depp’s job is less to show us what makes Jimmy tick—the film doesn’t particularly care—than to come on as implacably creepy as possible, and when Depp turns it on, he’s riveting. A scene in Connolly’s house demonstrates Jimmy’s power to intimidate, and Depp glories in the thinly-veiled threats of violence that he purrs across the dinner table. After he visits Connolly’s outraged and disgusted wife (Julianne Nicholson) to not-so-gently suggest she keep her mouth shut, you’ll want to run home and take two dozen showers.

This may seem strange to say with respect to a film that contains several murders, but Black Mass is actually less violent than I expected. The killings tend to be quick—executed with purpose and an offhand efficiency, befitting acts that everyone immediately proceeds to pretend never happened. This is business. Boys from Southie stick together.

– Jay Gabler