Johnny Depp ditches the flouncy flourishes, silly accents and funny hats and gets back down to the business of acting in Black Mass, easily his finest film in a decade. He disappears behind a science project of prosthetics, lenses and wigs to play the almost mythical Boston mob boss James "Whitey" Bulger.

Bulger is a figure so complex, criminally depraved and charismatic that he's been the subject of a Showtime series ("Brotherhood") and three movies besides this one: last year's excellent documentary Whitey: United States of America v. James J. Bulger; Whitey Bulger: The Making of a Monster (2015); and Martin Scorsese's Oscar-winning The Departed (2006), in which he was the basis for Jack Nicholson's over-the-top Frank Costello.

Director Scott Cooper (Crazy Heart) and Depp go in the opposite direction, opting for understatement and a gritty, '70s-style realism for the telling of a tale so outlandish that it would insult the audience's intelligence if it weren't the documented truth. Based on the 2000 book of the same name by Dick Lehr and Gerard O'Neill, the Boston Globe reporters who broke the story, Black Mass chronicles Bulger's rise from a small-time Southie hood to Boston's most notorious crime lord. That career trajectory was made possible by a secret pact with — are you sitting down? — the Federal Bureau of Investigation.

Joel Edgerton costars as FBI agent John Connolly, a childhood friend of Bulger who returns to the neighborhood with a proposition. In exchange for information that will aid the agency in rooting out the city's Italian mafia, he'll provide cover for Bulger and his Winter Hill Gang. The mobster is initially dubious but quickly comes to see the possibilities in Connolly's pitch. "An alliance like this doesn't weaken you," the agent promises. "It makes you stronger."

And that's precisely what it does. Connolly plays superiors and fellow investigators (Kevin Bacon and Adam Scott among the actors portraying them) for fools, claiming that Bulger is an invaluable asset. In reality, the mobster provides little useful information (but lots of dirty cash) and uses his free pass to expand his empire of extortion, drug dealing, numbers running, arms sales, murder and, for a brief time, the Florida jai alai industry.

Did I mention that Bulger's brother (Benedict Cumberbatch) was William "Billy" Bulger, the president of the Massachusetts senate? You can't make this stuff up. Can you imagine what Thanksgiving must have looked like for a family that contained both the most powerful figure in Boston politics and its most powerful crime figure?

We don't get that scene, but we do get a memorable snapshot of the brothers paying an early-morning visit to their mother's humble apartment. Billy cooks breakfast while Whitey agrees to a hand of cards, which Mom wins by fixing the deck. There's almost a tenderness in his voice when he folds and quips, "Thanks for cheating, Ma."

It's a humanizing moment in a character study filled with inhumanity and duplicity at their most extreme. Cooper, a former actor, ably elicits first-rate work from his extensive cast. Depp and Edgerton dominate, delivering masterfully calibrated performances. The script, credited to Mark Mallouk and Jez Butterworth, effectively compresses a potentially unwieldy span of time (the '70s through the '90s), and it does not suffer from a paucity of killer lines.

Early on, Bulger offers fatherly advice to his son, who's been punished at school for punching a classmate. He explains that he didn't get into trouble because he hit the kid, but because he hit him in front of other people. "If nobody sees it," Bulger explains, "it didn't happen."

Unbelievably, everything in Black Mass did happen. It's a hell of a movie. You should see it.