Imagine a dumbed-down version of “The Untouchables” done in floridly violent comic-book style and you’ll have some idea of what Ruben Fleischer’s “Gangster Squad” is like. Focusing on a covert police unit formed to bring down the Los Angeles empire of mobster Mickey Cohen in the late forties, its first victim is historical accuracy. Its second is any hint of good cinematic taste—a quality that actually worked to the benefit of Fleischer’s cheekily outrageous “Zombieland” (though not his awful follow-up “30 Minutes or Less”) but results here in a ridiculously glossy, hilariously cliched piece of movie pulp.

Will Beall’s script is based on the newspaper articles, and a book, by Los Angeles Times reporter Paul Lieberman, about a group of cops who were given extraordinary leeway to deal with the wiseguys, including Cohen, who were streaming into L.A. either as part of the dominant Jack Dragna gang or as free-lancers. But apart from that basic premise (and the names of a few of the mobsters and lawmen), the screenplay has very little to do with the actual record. Instead Beall has constructed a vigilante fantasy in which the squad, over what seems to be a few weeks rather than the years the actual bunch operated, succeeds in liberating their city from Cohen’s clutches and bringing him to justice.

It would be a tiresome business to catalogue Beall’s divergences from reality in constructing a story that’s a chain of shoot-outs, setbacks and triumphs. His model was certainly David Mamet’s 1987 update of the old Robert Stack TV series about Eliot Ness and his band of G-men pitted against the Capone gang in Chicago, with a dose of “L.A, Confidential” added to the mix. But his labored plotting exhibits none of Mamet’s skill in construction (or Brian Helgeland’s, who adapted James Ellroy’s book), and his dialogue is so riddled with noir cliché that one can only wonder whether it’s intended as a send-up of old gangster patois rather than a homage to the old flicks.

Fleischer is no less slavishly devoted to the “Untouchables” template. Despite lots of flash and visual bravado, however, he only proves that he’s no Brian De Palma–though not for lack of trying. He apes the older man’s style so insistently—in the scene portraying the (totally unhistorical) killing of one of the team member, meant to recall Sean Connery’s demise in the earlier picture, for example, or in the big final confrontation in the lobby of the Park Plaza Hotel, an attempt to do De Palma’s Union Station stairway sequence one better. But the comparisons are devastating. The last-act shootout, for instance, with its explosion of Christmas ornaments, is a chaotic jumble beside the earlier film’s scrupulously choreographed, balletic movement—though it’s not quite the incoherent mess that the director, cinematographer Dion Beebe and editors Alan Baumgarten and James Herbert make of a sequence early on in which the heroes attack one of Cohen’’ drug shipments in a frantic care chase.

As to the performances, they have more to do with posturing than acting. As John O’Mara, the bulldog leader of the squad, jut-jawed Brolin is so emotionally impassive that he actually rivals the wooden quality Robert Stack brought to the original TV Ness. (And the effort to soften him up by providing him with a concerned pregnant wife who’s threatened by the mob—a role better played by Mireille Enos than it deserves—doesn’t work.) He’s partnered with Ryan Gosling as Jerry Wooters, a guy as smooth and pragmatic as Brolin’s O’Mara is bluff and straight-arrow. Gosling apparently took the part to provide a respite from his customarily demanding roles. He glides through it nonchalantly, bringing little besides a slightly bemused expression. It’s hard, though, to see how anyone could have taken seriously a lachrymose scene in which Wooter’s persuaded to join the squad as a result of the death of a shoeshine boy in a gangland shooting. On the plus side, Gosling wears his period wardrobe well (as do all the cast, really), and he seems really to enjoy flicking his cigarette lighter on—something that happens so often it becomes a tiresome visual motif. And, of course, he gets to play scenes with lovely Emma Stone, doing her best imitation of a 1940s vamp as Grace Faraday, Cohen’s squeezes who begins an unwise romantic triangle with studly Wooters. It’s a ridiculous plot invention, of course, but no worse than most of what happens in the movie.

The rest of the squad is made up of good actors in hokey roles—Robert Patrick as a literal cowboy, Giovanni Ribisi as the nerdy but committed bugging whiz, Anthony Mackie as the black cop concerned about drugs in his neighborhood, and Michael Pena as the baby-faced Hispanic. Nick Nolte is hardly baby-faced as the chief who sets the squad in motion, but he’s no less a stock figure. And as Dragna, Jon Polito continues to play essentially the same character he’s been doing since “Miller’s Crossing.”

Then there’s Sean Penn. He plays Cohen—who was by all accounts a shrewd fellow who eventually, like Capone, went to prison for tax evasion (not murder, as here)—as a snarling animal with a propensity for exterminating any underling who makes a blunder in the most gruesome fashion (a tactic, one assumes, unlikely to engender much loyalty). Scrunching up his face in perpetual anger and contempt, Penn’s interesting to watch for a while, but by the close the performance has become rather tiresomely one-note. And unlike Robert De Niro’s Al Capone in “The Untouchables,” there’s an awful lot of it, including an absurd fistfight with Brolin’s O’Mara at the close. (Yes, the cop puts aside his gun to take on the villain—an erstwhile prizefighter—mano-a-mano. You didn’t expect Beall and Fleischer to bypass the ultimate cliché, would you?))

Technical credits are rich across the board, though Maher Ahmad’s production design, art directors Austin Gorg and Timothy D. O’Brien, set decorator Gene Serdena and costume designer Mary Zophres aren’t concerned with creating a realistic period look as much as a Hollywood dream of it. Nor—despite a few nods toward the issue raised by Ribisi’s character—is “Gangster Squad” at all interested in addressing the really intriguing issue raised by the story: do lawmen become indistinguishable from lawbreakers when they adopt their methods to bring them down? No “Bad Lieutenant” qualms here, just good old “Walking Tall” ass-kicking.

“Gangster Squad” was, of course, supposed to be released last year, but was delayed (and partially reshot) after the Aurora shooting. The original trailer—shown at the initial screenings of “The Dark Knight Rises,” ironically—featured a scene in which gunmen fired from behind a movie screen into a crowded theatre. That sequence is nowhere to be found in the final film, but it still will probably be what this movie will be remembered for, unlike “The Untouchables” and “L.A. Confidential,” which have lasted because of their quality. (One shouldn’t be too surprised if the cut scene eventually shows up on the Blu-ray, either) This jejune, testosterone-driven exercise in vigilantism won’t join those modern classics, but will doubtlessly go the way of another De Palma picture also loosely based on California crime history—2006’s opulent misfire “The Black Dahlia”—in the cinematic dustbin.