Legendary filmmaker Martin Scorsese follows up his 2004 Howard Hughes biopic "The Aviator" with a return to a world of crooks, thieves and murderers that he is particularly well-versed in. As such, and as one would expect, "The Departed" is a graphically violent, uninhibited, strictly adult affair. It also happens to be a pulpy, operatic, rousingly good crime thriller, one that goes on for two-and-a-half hours but has the juicy material, the slam-bang performances, and the almost epic scope to warrant every minute of it.

Leonardo DiCaprio (2002's "Catch Me If You Can") delivers a gritty and appropriately stressed-out turn as Billy Costigan, a young, blue-collar Bostonian whose dream is to become a police officer for the state. Instead, he gets offered an even more dangerous job that he can't refuse: go undercover as an ex-con and infiltrate the city's most infamous organized crime unit, led by savage gang leader Frank Costello (Jack Nicholson). Stripped of his real identity and putting his life on the line, Billy grows to know all too well just what will happen to him if his true intentions are uncovered.

Meanwhile, police chief Oliver Queenen (Martin Sheen) and right-hand man Dignam (Mark Wahlberg) suspect that a mole from Frank's camp is among their ranks, having joined the police force as a way of being the eyes and ears of the Irish Mafia. They'd be right; Frank's longtime apprentice Colin Sullivan (Matt Damon) is not only a newly appointed detective, but he has become the lead investigator into finding out who the secret informant is. It's only a matter of time before Billy and Colin's paths collide.

A remake of 2002's Hong Kong release, "Infernal Affairs," "The Departed" certainly earns its name, although how and in what ways will be left open for the viewer to discover. The film, written by William Monahan (2005's "Kingdom of Heaven"), opens quickly and wastes no time in setting up the premise, but it does take a while to find its rhythm. One of the biggest question marks permeating through much of the movie is what Billy's motivation is in giving up his life for over a year and going to the lengths he does to become one of Frank's own deadly cronies. Yes, he wants to be a respected cop, but by accepting this undercover assignment his identification is hidden, his safety is zilch, and the level of appreciation he gets in return from his superiors is virtually nonexistent. If one can accept the decisions Billy makes at the onset and recognize that the heart of the story is that of a young man who doesn't realize until it is too late how in over his head he is, "The Departed" kicks into high gear by the second hour and only gets better from there.

Without the loose ends of 2002's "Gangs of New York" and the safe and uneven storytelling of "The Aviator," "The Departed" may be director Martin Scorsese most complete and well-rounded effort since 1999's underrated and overlooked "Bringing Out the Dead." With someone of his experience and know-how at the helm, of course the film is a technically polished and well-made one. The editing by Scorsese regular Thelma Schoonmaker is lucid and quixotic, gliding with aplomb from one scene to the next and always keeping the pace moving. The cinematography by fellow Scorsese devotee Michael Ballhaus is unshowy but splendid nonetheless, building a thick, specific area feel to the Boston setting despite largely having been filmed in Manhattan. The soundtrack, from The Rolling Stones' "Gimme Shelter" to Pink Floyd's "Comfortably Numb," are used to support rather than overpower the story and images.

Where "The Departed" works best is in its playful skewering of conventions. The basic plot is rather ingenious, but its destination is one that would appear to be leading down a predictable path. Not so. By the third act, the film naturally and audaciously starts throwing surprise after surprise and unexpected plot development after unexpected plot development at the viewer, as if to stick its tongue out at cynical audiences who think but really have no idea where the movie is heading. Bristling with equal parts taut excitement and tense dread, Scorsese brings expertly staged sequences of pure suspense to the table, as in a sly chase between Billy and Colin on a city street with neither party wanting the other to see his identity, and a climactic scene set in an abandoned building that pulls off a sucker punch in its outcome.

Leonardo DiCaprio and Matt Damon (2005's "Syriana") are the respective leads, and are both excellent as they play characters who are complete opposites even as they must pretend to be like one another. DiCaprio, whose baby-face looks haven't really done him any favors in segueing into adult roles, makes up for his young appearance with acting talent of the highest order. He is always plausible as Billy Costigan, who becomes desperate to fulfill his duties while finding resentment in his relationship with the men of the law he is working for. Matt Damon has a tougher role as Colin Sullivan, who spends most of the film playing the false role of a good guy. What is amazing about Damon is that he is never likable even as he does likable things, his trustworthy exterior but a thin facade hiding a ruthless ugliness underneath.

Jack Nicholson (2003's "Something's Gotta Give") owns the part of heavy Frank Costello to the point where this should go down as a defining role for the actor alongside 1980's "The Shining," 1989's "Batman," and 2002's "About Schmidt." Nicholson is in full-on crazy mode as Frank, but he carries it with such an evil glee, maniacal sneer and characterization specificity that it is unlike anything he has done before. Black humor runs through Nicholson's scenes, and yet he never wades into campiness; it's plainly clear that this is a threatening and psychotic man. As psychiatrist Madolyn, who gets romantically involved with Colin as she takes Billy on as a patient, Vera Farmiga (2006's "Running Scared") is memorable in the only female role of any consequence, making what could have been thankless into something more poignant and layered. As Sgt. Dignam, Mark Wahlberg (2006's "Invincible") does well with an underwritten character whose hostility wears on the viewer's nerves just as it does on his coworkers at the police station.

"The Departed" is a mature and entertaining crime drama with a Hitchcockian flair and an ability to divert expectations in creative ways. Shaved of its style and A-list pedigree, the film doesn't have a whole lot of substance—the hard-boiled story, while original, is more suited as popcorn fare for older audiences than thoughtful cinema—but that's more than all right under such mesmerizingly mounted circumstances. "The Departed" isn't afraid to be dark, morbidly funny and shockingly violent in abrupt spurts. The great final shot, leading to a cleverly symbolic zinger, manages to combine all three. It puts a fitting capper on a motion picture that may be a remake, but has that magic Scorsese touch that sets it apart from both its precursor and its genre as a whole.