At one seemingly innocuous point in Scott Cooper's visceral crime drama Black Mass, notorious mobster James "Whitey" Bulger (Johnny Depp) tells his son, who has been dealing with a bully, that he didn't act wrongly by retaliating (punching his aggressor in the face); his mistake was getting caught. The scene is played for levity (Dakota Johnson's aggrieved reaction as the boy's mother is excellent), but Bulger's advice doubles as a summary for the plot that drives this highly charged yet slow-burning film. Once a small-time crime boss in Boston as the leader of the notorious Winter Hill Gang, Bulger leads a charmed career. His younger brother Billy (Benedict Cumberbatch) is an established local politician who eventually works his way up to the position of state senator. In addition to the protection that his family ties afford him, Billy's childhood friend John Connolly (Joel Edgerton), now a hotshot FBI agent, comes to him with a mutually beneficial proposition: Connolly is trying to remove Italian-mob crime boss Jerry Angiulo (Bill Haims) from power, and offers to take the heat off Bulger in exchange for information on his "business rival." Bulger obliges at first, which leads to Angiulo's arrest and a major profile boost for Connolly, who enjoys both his increased power at the Bureau and the newfound swagger that comes from palling around with fellow "Southie" Bulger. Unfortunately for Connolly (and the FBI and the city of Boston), Bulger unsurprisingly stretches the wide leash he's been given and runs with it; he continues to perpetrate major crimes, including arms smuggling, drug running, and high-profile murders, all while implicating Connolly in the process. The Winter Hill Gang's menace is contrasted with their extreme lack of subtlety, and the true-life story builds toward the downfall of Bulger and Connolly. Police interrogations involving two crooks with everything to lose -- Winter Hill members Steve Flemmi (Rory Cochrane) and Kevin Weeks (Jesse Plemons) -- provide the framework for the action-packed plot of Black Mass. Director Scott Cooper (Crazy Heart) takes the visual and emotional grit of his last picture, 2013's Out of the Furnace, and applies it to this successor to such standout inside-the-mob films as Scorsese's Goodfellas and Boston-centric The Departed. Cooper keeps the movie simple and sticks to the script: We see the gangsters and the powerful men in their orbit over the course of 1975 to 1986, as the storytelling relies as much on simmering tension and frayed nerves as it does on actual violence. Two standout scenes include an over-the-shoulder perspective of a ferocious crime that doubles as a turning point in the film, and a sizzling workplace altercation between Connolly and the put-upon boss who gave him permission to collaborate with Bulger in the first place. These scenes cleverly juxtapose law enforcement and the criminal underworld, showing how the same sense of authoritarian overreach and greed exist in both settings. Cooper's skilled direction gives us a detailed look at these seemingly different milieus, and the understated, vibrant dialogue from screenwriters Mark Mallouk and Jez Butterworth (made possible by the acclaimed book of the same name from journalists Dick Lehr and Gerard O'Neill) elevates the film from compelling to superb. Cooper, Mallouk, and Butterworth deftly avoid romanticizing the rampant criminal violence, choosing instead to let the conflict between these powerful, flawed men serve as the movie's axis. As a result, it's largely up to the actors to lend this true-life film the emotional depth it deserves, and the high-wattage cast are more than up to the task. Depp is sensational as Bulger, utterly captivating without ever coming across as ostentatious. With the help of extensive makeup work and an icy stare intensified by the shade of blue in his contact lenses, the A-list star manages to disarm us with only a smile, and chill viewers to the core with a twitch of his eyebrow. Despite an acting career that's among the most intriguing and versatile of his generation, Depp's work here stands as one of his most fully realized performances. The rest of the cast are aces as well. Joel Edgerton portrays Connolly as a man with a chip on his shoulder who is brought down by his ambition, misguided loyalties, and the indignant rage he carries with him. After anchoring a handful of affecting scenes early on, Plemons fades into the background for the majority of the film, but he manages to make the most of his screen time. The other great supporting performances include Peter Sarsgaard as a loose-cannon drug addict tangled up with the wrong people, and Cochrane as Flemmi, whose sad eyes betray the broken sycophant behind the menacing enforcer. Certain scenes are delivered with an excess of melodrama, which is rendered unnecessary by the high-quality cinematography by Masanobu Takayanagi. In addition, the able performances of Johnson and Julianne Nicholson (as Connolly's wife Marianne) are glossed over for the vast majority of the movie. It speaks to how compelling the story is that viewers will leave craving more from certain characters, including Kevin Bacon as federal agent Charles McGuire and Cumberbatch as a man whose political hopes are undermined by a warped sense of familial duty. This film is sure to garner awards attention, but it also has the gravitas and across-the-board craftsmanship to stand the test of time as a killer true-crime tale.