In "The Departed," South Boston mob kingpin Frank Costello (Jack Nicholson), left, consults trusted insider Billy Costigan (Leonardo DiCaprio), not knowing that Costigan's really an undercover cop helping the Massachusetts State Police blow the whistle on Costello.

Welcome home, Martin Scorsese.

It's been more than a decade -- since 1995's made-in-Vegas, twilight-of-the-mob "Casino," in fact -- since you've hit the mean streets that have been your cinematic home since, well, 1973's "Mean Streets."

Through the years as one of filmdom's best and most passionate filmmakers, Scorsese has roamed far and wide, delivering classics as different as the blistering boxing drama "Raging Bull" and the hauntingly spiritual "Kundun" and "The Last Temptation of Christ."

Yet he's always had a special touch when he brings things down to street level, whether the protagonist is a lone "Taxi Driver" or the mobbed-up "Goodfellas" strutting around like the toasts of the town.

In "The Departed," the setting shifts from Scorsese's native New York to Irish-dominated South Boston, better known as Southie.

Despite the geographical switch, however, "The Departed" demonstrates that Scorsese knows this territory by heart -- or, more precisely, by blood.

The change of scene seems particularly apt for a movie inspired by 2002's "Infernal Affairs," an infernally intriguing Hong Kong thriller.

Adding numerous twists to its already convoluted source, "The Departed" preserves its inspiration's central premise, the cat-and-mouse -- or, more precisely, cat-and-rat -- chase between two crafty young players working both sides of the street.

One's an undercover operative who's a trusted member of a top gangster's team. The other's part of an elite team out to bring the gangster down. It's an unlikely prospect, because the star investigator is working undercover himself -- for the gang kingpin.

The two movies share the same premise, but "The Departed" isn't so much a translation as a transformation.

Using the sleek, streamlined original as a springboard, Scorsese and screenwriter William Monahan ("Kingdom of Heaven") deepen and expand the material to explore the essential dualities of human nature -- and the inescapable consequences of same.

But don't despair. (Even if "The Departed's" characters sometimes do -- at least the ones who still have a shred of conscience left.)

Beyond its dizzying dualities and blood-spattered examinations of right and wrong, "The Departed" turns out to be a literal blast, full of nervy, live-wire characters threatening to short out -- or spark bullet-riddled mayhem -- in the flash of a gun.

They're like loose balls in a perpetually tilted pinball machine, bouncing around (and up against each other) as "The Departed" shifts between action and absurdity -- or mixes them up, generating the kind of gleeful havoc where you're laughing one minute, then choking as the laughter freezes in your throat.

And that's just the way Frank Costello likes it.

Southie's patron saint (or resident evil, depending on your perspective), Costello (Jack Nicholson) has his finger in a variety of profitable but predictably illegal pies, from drugs to porn to stolen computer chips.

Little wonder that he's the top target of the Massachusetts State Police -- home of rising departmental star Colin Sullivan (Matt Damon), a sharp son of Southie who's been in Costello's pocket since he was a kid.

Unbeknownst to Sullivan, he's got a counterpart in Costello's inside circle: trigger-tempered Billy Costigan (Leonardo DiCaprio), who's spent his life trying to live down his family's criminal past and Southie roots.

That makes Costigan a perfect undercover candidate in the minds of avuncular Capt. Queenan (Martin Sheen) and testy Sgt. Dignam (scene-stealing Boston native Mark Wahlberg), the only two cops around who know Costigan's not mob muscle.

Sullivan and Costigan look enough alike to be brothers. Considering their hardscrabble Southie backgrounds, they could be brothers -- or cousins.

Instead, they're doppelgangers -- and moles, working the same labyrinth from opposite ends, burrowing toward the middle and the inevitable showdown destined to end in floods of blood. (Hey, whaddya want, a Boston tea party? It's a Scorsese gangster movie -- how else could it possibly end?)

Their very different balancing acts -- Damon's smoothly duplicitous, DiCaprio's anguished and desperately dogged -- give "The Departed" a galvanic, push-pull energy that sharpens the characters' connections.

In this game, though, everybody gets a chance to score, whether it's Alec Baldwin's hilarious vulgarity as a cut-the-bull police supervisor or Ray Winstone's eye-of-the-hurricane intensity as Costello's chief enforcer. And Nicholson, especially before he suffers a minor Jack attack in the movie's final scenes, reins in his trademark mannerisms, but not Costello's equal flair for the dramatic -- and the traumatic.

Behind the camera, Scorsese's all-star team (led by cinematographer Michael Ballhaus and editor Thelma Schoonmaker) flexes its collective muscles, enabling the director to maintain the story's suspense and momentum (not always Scorsese's strongest suit) even as he details the endless ways his cops and crooks -- and crooked cops -- betray each other. And themselves.

Getting down and dirty may not be good for the soul. But, clearly, it's good for the director. Especially when that director is Martin Scorsese, back where he belongs -- at the top of the heap, working at maximum impact.