We already got a “Step Up” movie in 3D two years ago, and another certainly was not needed—especially one as trite as “Step Up: Revolution,” which not only lacks anybody with the charisma of Channing Tatum, who gave the original whatever pizzazz it possessed, but much of anything else beyond a string of strenuous dance numbers.

Despite the subtitle, the script seems peculiarly outdated in focusing on the flash mob craze which, unless I’m mistaken, peaked some time ago. In particular it centers on a Miami group of underground dancers who call themselves—with a lack of imagination that happily doesn’t extend to their routines—The Mob. The Mobsters, if you’ll excuse the term, include regular members who are identified in a sort of playbill—the videographer, the parkour specialists, the DJ and so forth. But the only ones who emerge as distinct characters are Sean (Ryan Guzman), the lead dancer, and his best buddy since childhood, Eddy (Misha Gabriel), who’s the chief computer guy but apparently doesn’t hit the floor himself. The two work regular shifts as waiters in a fashionable restaurant—though frankly they look pretty scruffy and disheveled for such work—only to earn enough to support their dance efforts. Their goal, you see, is to have their elaborate guerrilla routines, which they post on YouTube (product placement alert!), be the first to reach ten million hits, which would win them a hundred thousand dollar grand prize. Unfortunately, they’re running second to a singing cat.

Hunky Sean, who lives with his pragmatically-minded sister and her adoring little daughter, meets pretty Emily (Kathryn McCormick) in a sort of beach-party dance-off, and it’s clear they’re both smitten. What he doesn’t know is that she’s the daughter of development mogul Bill Anderson (Peter Gallagher), who’s just bought the place where he works and canned Eddy for tardiness (rightfully, one might point out). She’s intent on winning an internship at a prestigious modern dance school—a dream her dad thinks wildly impractical. Before long she joins The Mob and stars in some of their finest pieces of performance art. But things get messy when her daddy buys up the neighborhood where Sean and Eddy grew up, and where they and the community congregate in a raucous saloon called Ricky’s, and The Mob takes to performing numbers designed to protest—and impede—the devastating development he plans there. When Eddy, peeved that his bro shows more concern for Emily than him, publicly outs her, it threatens to destroy their romance, and her dance dreams. But rest assured—all will work out in the end, thanks to a Mob performance to outdo all their previous ones.

Considered from the standpoint of even the most rudimentary logic, “Revolution” is completely negligible. The “let’s put on a show” underpinning is as hoary a device as one could imagine, and plugging it into the Internet ethos doesn’t make it any less musty. But even if one buys into it, the notion that The Mob could create such elaborate numbers overnight and execute them so flawlessly, without every getting hassled by the cops or irate passersby, who would be incredibly inconvenienced by them, is absurd. And the idea that a dance number, however extraordinary, could incite a proletarian reaction against urban renewal and save a neighborhood is beyond far-fetched. The scenes with Mia Michaels as Emily’s dance guru, moreover, are totally ludicrous.

The movie is also dreadfully acted. Guzman, a model, and McCormick, a dancer, can barely recite their ultra-hokey lines, let alone make them remotely credible. Gabriel is even worse—his firing scene is hilarious—though he does look right cool wearing his hat at an angle. Only the veteran Gallagher gives a hint of a real performance, though it’s of after-school special quality.

And yet the dances are energetic and imaginative, especially one set at a posh restaurant and another at an art gallery, and all are staged artfully for 3D, though the parade of gyrating buttocks actually gets tiresome after awhile. Though director Scott Speer can’t give the expository scenes any life, he handles the musical numbers well, and you have to hand it to the choreographers (Jamal Sims, Christopher Scott, Travis Wall and Chuck Maldonato) and cinematographer (Crash) for the staging as well, even if The Mob tends to expand miraculously whenever they need more bodies in front of the lens. Music supervisor Buck Damon also deserves a nod.

But this fourth installment of the series is still several steps below the original—which was no great shakes to begin with—and unless energetic demonstrations of terpsichore are enough to satisfy you, you’d best steer clear of The Mob.