His character's receding blond hair slicked back, his eyes an icy, vindictive blue, Johnny Depp vanishes behind the shuddersome visage of infamous Boston mob boss James 'Whitey' Bulger in chilling crime docudrama "Black Mass." It is impossible to deny Depp's range as an actor, but with a career that has in recent years been predominately dotted with broad, flamboyantly quirky parts like the rum-swilling Captain Jack Sparrow in the "Pirates of the Caribbean" franchise, maladjusted candy maker Willy Wonka in 2005's "Charlie and the Chocolate Factory," the unhinged Mad Hatter in 2010's "Alice in Wonderland," scorned vampire Barnabas Collins in 2012's "Dark Shadows," Camanche spirit warrior Tonto in 2013's "The Lone Ranger," and the bumbling, ridiculously mustachioed title art dealer in 2015's "Mortdecai," it takes a subtler, grittier, more earthly role to remind just how beautifully he can reel back the outward showmanship and dig deep within.

Directed by Scott Cooper (2013's "Out of the Furnace") with the vengeful precision of an assassin's scope, "Black Mass" traverses the tangled, unsettlingly dark days during the 1970s and '80s when Jimmy Bulger played both sides, manipulating the FBI as an informant against the Angiulo mafia family while running an Irish mob crew called the Winter Hill Gang. Straddling both sides and protecting what he knows are nefarious dealings involving racketeering, money laundering, extortion and murder is tight childhood pal John Connolly (Joel Edgerton), an FBI agent torn between his professional duties and his loyalty to Jimmy. For a long while, Jimmy's cooperation with the Feds grants him immunity from criminal activities the Bureau chooses not to acknowledge, but as his dirty deeds pile up, the truth about his crooked connection with federal, state and local law enforcement starts catching up with all involved.

Based on Dick Lehr and Gerard O'Neill's 2013 true-crime book "Whitey: The Life of America's Most Notorious Mob Boss," "Black Mass" takes dramatic license when it comes to certain characters and timelines—a fact that will only be apparent to viewers who are knowledgeable of James 'Whitey' Bulger's past prior to seeing the picture. Judged strictly on its own merits, this is never less than riveting, adult-minded entertainment. Screenwriters Jez Butterworth (2014's "Edge of Tomorrow") and Mark Mallouck spin a grimly immersive saga unapologetic in its cold-blooded viciousness. More sprawling snapshot than intimate character study, the film focuses most closely on Bulger and Connolly's relationship, and how their lives are affected after the former agrees to be an informant for the FBI while continuing to cast his own separate reign of terror on anyone who proves an untrustworthy liability. Following the advice he gives 6-year-old son Douglas (Luke Ryan)—that he can do whatever he wants as long as nobody's looking—Jimmy ramps up rather than slows down the Winter Hill Gang's corruption, correctly believing for years that the Feds are concentrating on every direction but his own.

As a man who cares deeply for his family—he never fully recovers from the losses of his elderly mother and young Douglas, whose life is cut short by Reye's syndrome—but can flip on a dime to become a real-life boogeyman, Johnny Depp is simultaneously charismatic and scary as James 'Whitey' Bulger. Disappearing behind his quietly evil but far from one-note character while barely raising his voice above a whisper, Depp's transformative performance edges close to career-best territory. Indeed, he is such a force in this role that one longs for him to be on the screen even when he is absent. Told through a framing device where Bulger is discussed by longtime accomplices being questioned by federal officials, the story does not dig as deeply as it could have into its subject's mind and consciousness. Because much of what is learned of Bulger is spun from third parties, Depp's layered turn serves to fill in the gaps.

Gravitating around Depp is an exceptional ensemble. Joel Edgerton (2015's "The Gift") has a difficult job to pull off as Agent John Connolly, a South Boston native who reenters Jimmy's life for business purposes and gradually becomes hypnotized by the allure and material promises of his friend. A sympathetic figure overshadowed by wayward allegiances, Connolly shifts from early protagonist to a provocative gray area. Julianne Nicholson (2013's "August: Osage County") finds courage and vulnerability in John's wife, Marianne (Julianne Nicholson), a woman who senses she is losing her husband and has no way of stopping it. A scene where Bulger sizes Marianne up while quietly menacing her at her bedroom doorway is beyond intimidating, breathlessly suspenseful in its stillness.

Juno Temple (2014's "Horns") makes a lasting impression in her small but unshakable appearance as Deborah Hussey, the loose-lipped prostitute-stepdaughter-girlfriend of Jimmy's right-hand man Steve Flemmi (Rory Cochrane). As Jimmy's wife Lindsey, Dakota Johnson (2015's "Fifty Shades of Grey") shows up for a handful of early scenes. Hers is a presumably thankless role until she comes out swinging in a heated confrontation with her husband driven by heartbreak and, as Jimmy sees it, betrayal. Benedict Cumberbatch (2013's "The Fifth Estate") is aptly cast as Jimmy's fiercely loyal senator brother Billy, exhibiting shifty, stern and ingratiating shades. Jesse Plemons (2012's "The Master") is virtually unrecognizable as Jimmy's trusted confidante Kevin Weeks, a result of far more than aging makeup. Tremendously effective in a part very much at the opposite end of the parts he usually receives, Plemons hasn't been given the chance to impress this fully since his days on TV's "Friday Night Lights."

"Black Mass" is a grievous, blood-strewn, sumptuously lensed portrait of a human monster who believed he was above the law—and, for far too many years, kind of was. Preaching about loyalty and the bonds of brotherhood even as he hypocritically deceived the majority of his associates, James 'Whitey' Bulger is an enigmatic figure embodied by Johnny Depp with a compelling intensity all the more glorious for how restrained he plays it. He is the main attraction in a film that, in tone and milieu, has accurately been compared to Martin Scorsese's 2006 crime thriller "The Departed." Director Scott Cooper opts for mood and taut narrative rhythms over substantive dives into Bulger's twisted psyche—a valid creative choice that keeps his subject partially shrouded in mystery. "Black Mass" exposes enough of his ghastly crimes (he was convicted of 11 murders and thought to be complicit in up to 19) to send the sobering message that what this man was capable of is likely beyond the realms of understanding and comprehension.