YOU'RE NEVER sure what you're watching in "The Untouchables." There's an entertaining but incongruous mix of class and pulp. Director Brian De Palma gives you an esoteric tribute to "Battleship Potemkin" -- a Russian film of the 1920s -- as well as a dip into the pulp-lore of the old Warner Brothers gangster flicks. He gives you "The Magnificent Seven Wear Armani," and then a period "Roadrunner."

De Palma and screenwriter David Mamet have doctored Eliot Ness's true account of his battle against Al Capone's empire into a facile Hollywood pantomime, where you hiss and boo at the Mob and cheer the four Fearless Cops just doin' their job. The gritty story of 1930s Chicago becomes grist for the 1980s Lite-legend mill. Just the heroics, please, Ma'am.

The true story certainly invites this treatment. Al Capone really did rule Chicago, his bloody hand in everything -- even had a scarred face. And as played by the inimitable (and pudgy to order) Robert De Niro, he's a leering smoothie with a Mussolini swagger. In a scene cribbed from the baptismal murders in "The Godfather," he orders an assassination while crying ethnically at the opera. He's a bum in expensive clothing -- slugs his lawyer when his trial goes awry and beats a flunkie's brains out at an elegant dinner party. You didn't think De Palma would miss out on this stuff did you?

Eliot Ness, as portrayed in his 1957 book "The Untouchables," was American Gothic-straight, determined to wipe out crime with a near-messianic sense of purpose. He was America's straitlaced Gordon of the Nile, or Robert Baden-Powell. For Ness, if it was the law -- good law or bad law -- it had to be obeyed.

It's therefore superfluous to criticize Kevin Costner's Ness for being a stick-in-the-mud. But there's little about this fictional hero to empathize with. Although there's a tornado of murderous corruption around him, he's a calm, dull eye of the storm. Mamet gives Ness a wife and children, presumably to warm his character up. But as husband and father, Ness is only half there -- a blow-up human. "Sure is nice to be married," he keeps saying. You wonder why.

As a Treasury Department agent assigned to break Capone, he sleepwalks into a palm-greasy town, naive as a Boy Scout. But he enlists veteran Irish cop Jimmy Malone (the Scottish-accented Sean Connery) as a benevolent Svengali, to show him the ways of Windy Hades. They recruit a streetwise, sharpshooting Italian cop (Andy Garcia) and a gimpy, four-eyed accountant (Charles Martin Smith) to their ranks and they become a sort of fab four who can't be bought -- hence "the Untouchables." And the shootin' begins.

While Mamet gives the four characters some moments of interesting buddy-banter -- the underrated Connery provides the human power here -- his characters are melodramatic plot agents (mother of slain child, corrupt cop, hood). And some of De Palma's shoot 'em up and chase scenes, while initially exciting, wear out their welcomes because De Palma tends to render everything into movie-brat rhapsody. Things seem to drag lushly on forever. And although he can make a scene explosive with dynamic editing and overhead camera angles, after a while you may start to think you're a voyeuristic -- and very dizzy -- seagull. THE UNTOUCHABLES (R) -- At area theaters.