When I first heard that Martin Scorsese was remaking "Infernal Affairs," the ingenious 2002 Hong Kong thriller about undercover informants on opposite sides of the law, I had (quite fittingly) two opposing and equally weighted reactions. I was relieved and excited that after "The Aviator" and "Gangs of New York" — two period epics that, whatever their considerable merits, were clearly pitched more toward Oscar voters than audiences — Scorsese was returning to the criminal underworld where he belonged.  Yet I was also worried: Why remake what was, to my mind, just about a perfect movie?  Would Leonardo DiCaprio and Matt Damon be able to reproduce the hotshot charisma of Tony Leung and Andy Lau, and should they even bother trying?  Did this Asian cops-and-robbers classic really need a white-boy doppelganger of its own?

Fortunately, Scorsese knows a good thing when he sees it, and he and screenwriter William Monahan ("Kingdom of Heaven") have approached "Infernal Affairs" with both consummate respect and their own fierce, and fiercely original, interpretation. If  "The Departed" is easily Scorsese's strongest picture in years, and the meatiest thriller to come along in many a moon, it's because the filmmaker has both upheld and, well, departed from his model, and in all the right ways. 

Blessed with one of the most structurally and emotionally fertile thriller premises imaginable (a cop pretending to be a gangster, a gangster pretending to be a cop, both rats trying to sniff and snuff each other out), Monahan has chosen to stay faithful to the original story, largely replicating its devious twists, double-crosses and existential paradoxes.  Yet by relocating the action to the grimy bars, flats and warehouses of South Boston's mob scene, he has steeped it in a wholly appropriate bath — a bloodbath, if you will — of Irish-Catholic fatalism.  Where "Infernal Affairs" was an elegant labyrinth of cool, glassy surfaces and urban-luxe intrigue, "The Departed" is every inch a creature of the gutter and proud of it; it's a robust, red-blooded American entertainment.  It's no coincidence that halfway through the film, Costello is shown attending an opera performance (a mere prelude to an interracial ménage à trois), for this is violence and mob rule heightened to furious, operatic levels.  The score may be borrowed from Alan Mak and Andrew Lau, but the vulgar music — the profanity, the pulse, the pulp poetry — is pure Scorsese.

The ironies start out fast and thick at the outset and never stop piling up. Colin Sullivan is first introduced as a young, fresh-faced boy who is plucked and nurtured by powerful crime lord Frank Costello (Jack Nicholson) and eventually grows up to be played by Matt Damon, whereupon he swiftly works his way through the upper echelons of the state police department.  He may be scum, but he's privileged scum, a man who enjoys a classy date and a penthouse view.  Concurrently, while scrappy Bill Costigan Jr. (DiCaprio) is trying to be an honest cop, he comes from much poorer stock, a family with a long history of run-ins with the law.  Which makes him just the guy to attempt to infiltrate Costello's inner circle and bring him down.

The Costello role, greatly expanded from the original, gives Nicholson a prime chunk of scenery to chew on, enabling the actor to indulge his inner Jack to an unprecedented degree (witness his imitation of a gnawing rat) and, of course, give his eyebrows a devilish workout.  It's a lip-smacking performance that at times threatens to eat the movie alive, although Nicholson, for all his swarthy menace, is not the most interesting person on the screen.  Damon is aces as the guarded, calculating Sullivan, limning his inner dilemma with every ferret-like shift of the eyes.  It's the character's ferocious intelligence, not his morality, that turns us on.  And as his alter ego, DiCaprio, who came tantalizingly close to greatness as Howard Hughes in "The Aviator," fully holds his own here with a slow-burning performance that radiates both sociopathic energy and spiritual desolation.  If the actor has often seemed like a boy striving to fit in among men, he shows none of that strain here, despite (or perhaps because of) the fact that his Costigan is essentially stuck in the same dilemma.  A good mole must be a good actor, and DiCaprio establishes himself here as one of our best.

Even in "Infernal Affairs," Leung's broody undercover cop was the most poignant and tormented figure — apart from his superior, the only other person who knew his real identity, he was truly and completely alone.  In "The Departed," Costigan reports to two men, played by Martin Sheen and a hilariously surly Mark Wahlberg, a choice that automatically dilutes some of the tension and the emotion.  Intriguingly, the two female significant others in the original have been compressed into one, played by the terrific Vera Farmiga, a daring structural gambit that pays off beautifully in the closing reels.

The final twists of "The Departed" are spelled out in blood — great, gushing geysers of it that leave no doubt about Scorsese's mastery as an orchestrator of screen carnage.  The director has always been a genius at conveying not only the brutality of violence, but also its horrible matter-of-factness, the way it can erupt at random.  If the narrative threads don't ultimately converge with the economy and finesse of "Infernal Affairs," or the same shocking onrush of feeling, it's because that film's sensibility was genuinely tragic where Scorsese's is ultimately caked in cynicism; he knows this world too intimately to look for any catharsis in it.  "GoodFellas," tough and ruthless and brilliant as it was, had the same problem.  That I can even mention "The Departed" in connection with "GoodFellas" should be taken as recommendation enough.