This is what you want in a Martin Scorsese film: beautifully edited, brutally violent sequences, brimming with life even as bodies are hitting the floor, all awash in a blaring Rolling Stones tune. (In this case, ?Gimme Shelter?. Again.)

While ?The Departed? is an Americanised version of the 2002 Hong Kong hit ?Infernal Affairs?, it?s vintage Scorsese ? for a while at least. The veteran director has made two-thirds of a great film about Boston cops and mobsters, with dazzlingly rich performances from a dizzyingly stellar cast and an ambience that screams Scorsese?s typical cultural authenticity. (It?s as if the fellas from ?GoodFellas? took a road trip up I-95.)

Leonardo DiCaprio, reuniting with the director for a third film following ?Gangs of New York? and ?The Aviator?, stars as Billy Costigan, a Massachusetts State Police detective who?s gone undercover to take down crime boss Frank Costello (Jack Nicholson, in a devilish tour de force). Matt Damon, meanwhile, stars as the crime boss? protege, Colin Sullivan, who?s worked his way up the state police ranks to become staff sergeant of the special investigations unit.

The two function in a perfect parallel until it starts to become clear that Costello?s gang is staying one step ahead of the police, even while the police seem to know too much about Costello?s operations.

Never has the use of cell-phone text messaging seemed so sinister.

And so Billy and Colin are each asked to sniff out the rat in their midst ? to seek out each other.

As in the original, it?s a nice touch that the two actors so strongly resemble each other, with those crystal blue eyes and a look that can either be considered boyish or fiendish, depending on your perspective.

Billy and Colin are two sides of the same coin, both raised in South Boston, both trying to escape their lineage of Southie Irish trash but having chosen opposing routes.

(Screenwriter William Monahan grew up in the city and draws strongly from his roots. He absolutely gets this place, its people and rhythms, leaving you feeling immersed. It also helps a great deal that Scorsese shot much of the picture on location, and that Damon and co-star Mark Wahlberg are natives. At least you know they?ll get the accent right.)

It?s a clever premise and it can be thrilling, but ?The Departed? is also about a half-hour too long, and tends to drag just when it should be at its most intense.

A movie like this should grab hold of you and never let go; ?The Departed? does just that in its set-up and as the cat-and-mouse game begins, with several supporting actors infusing the film with a brash energy.

Wahlberg, as an eloquently surly sergeant in the detective unit, steals every scene he?s in; Alec Baldwin, as the jacked-up police captain, is just as darkly funny. On the other side, the underappreciated British character actor Ray Winstone is a force of nature, as always, as Costello?s top thug.

And Vera Farmiga provides a touch of warmth as a police psychiatrist and the only woman among the main cast ? though it is a massive plot contrivance to have her character get involved with both Billy and Colin.

But mostly the film belongs to Nicholson, as they all do whenever he?s involved.

The part is tailor-made for him ? all bluster and swagger, with bottomless menace beneath the suave smile, and it?s a hoot to watch him unravel as the film evolves.

Their work, and that of Martin Sheen as the level-headed captain who places Billy undercover, protects his identity and helps keep him sane, can only sustain a film for so long when it?s bloated.

?Infernal Affairs? remained taut throughout its 101 minutes; ?The Departed?, at 150 minutes, sort of lolls around awhile, with lots of soul-searching and pill-popping, before reaching its climactic rooftop conclusion.

Ultimately, though, what began life as a classic Scorsese film nearly morphs into self-parody, with characters literally standing around, waiting to get shot in the head.

This is the problem when you?re good at your job: People come to expect nothing short of excellence from you, and they?re disappointed when they receive anything less.