Returning from the latest cinema shoot-up usually is not cause for introspection. But they're not all made by Martin Scorsese, and you usually don't enjoy them this much. "The Departed" is a vigorous, though conventionally furbished, police thriller, extremely entertaining but perplexing.

Scorsese said after "The Aviator" that he was done with big-budget films. But here he is with an Entertainment Weekly's worth of big names, working with material that is in lock step with Hollywood's current obsessions.

As it is, the screenplay -- adapted from a Hong Kong picture called "Infernal Affairs" — isn't first-rate. But Scorsese elevates it, keeps it moving with little overt artifice except the same kind of poetic violence that made "Goodfellas" such a classic.

The story focuses on two fresh-faced Irish cops from the mean streets of Boston, both working for Irish mobster Frank Costello. One is golden boy Colin Sullivan (Matt Damon), a glib state police graduate serving as an inside guy for Costello. The other is Billy Costigan (Leonardo DiCaprio), an academy flunky recruited to infiltrate the crime boss's crew.

The director shows his expertise early with a swift introduction. The camera is fluid and the cuts are rapid-fire as we learn about police work, the local crime hierarchy and the antagonistic history of both, and Scorsese quickly anchors somewhat shaky plot points with firm direction.

It's slightly implied that Sullivan's father lost his life in some service to his boss, who takes him in without any explicit reason. We don't learn much about Sullivan's motivation except that he likes the easy money and believes Frank's philosophy that there's not much difference in who he works for. Costigan gets a little more time, and the difference reinforces the notion that more is not always better. It might be important to know that Costigan's a guy who can score 1400 on his SATs yet talk like a Southie street tough. But the assertion by his bosses, Captain Queenan (Martin Sheen) and Sergeant Dignam (Mark Wahlberg), that he's perfect for the job because he'd make a terrible cop made me think, "Huh?"

There are other sources of befuddlement. Watching Queenan and Dignam repeatedly inflict the good-cop/bad-cop routine on their operative, for example, is not the most original of cop movie routines, and it doesn't even make a lot of sense. It further strains credibility to see Sullivan and Costigan meet and fall in love with the same shrink, and Scorsese, lowering himself to the ignoble love scene, doesn't help.

In any other hands, this material could easily have degenerated into a similar but goofier concoction like John Woo's hit "Face/Off." Scorsese can't turn it into "Goodfellas" either, but his fluency with the medium is such that he can shock, titillate, frighten and make an audience laugh, seemingly at will and sometimes all at once, no matter the material. He also has a lot of help from his actors.

DiCaprio in particular is good as a lost soul growing more desperate as he engages in crimes for the purposes of information and evidence. In this characterization, desperate, lonely and yet determined, he further distances himself from the heartthrob tag he acquired early in his career. Matt Damon is just as convincing as a back-stabbing dirty cop, much more believable as a matter of fact than he was as the white knight he played in his "Bourne" movies. Nicholson, with little apparent effort, manages to make Frank scarily, psychotically violent; the real work seems to have been in not overdoing it, and he doesn't, mostly. He has his wacky Jacky moments, but they have a point.

Still dizzy from images of shootouts, beat-downs and foot chases of the back-alley, cloak-and-dagger variety, I wandered out of "The Departed" like I'd just gotten off a thrill ride. Movies like this operate with the same notion: that it's fun to experience pretend death. Characters drop like dominoes at the end of the last act. This swift, unsympathetic end is perhaps what Frank is talking about early in the film when he's pontificating wise guy to young Sullivan about cops and hoods. "When there's a gun to your head," he asks, "what's the difference?" I'm still not sure exactly what he means, or that it means all that much to the movie. The only difference here is Scorsese, who makes the bullets and the banter zing with entertaining zeal. (R) 152 min. **** S

The Quick Flicks short reviews of curent films can be found at www.styleweekly.com. S

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