How can a movie this bad be so good? “Step Up 2: The Streets” is a Franken-film, stitched together from the corpses of other dance movies: It’s got the legs of “Stomp the Yard,” the kidneys of “You Got Served,” the arms of “Save the Last Dance,” and the kneecaps of “How She Move.”

The clichés, in order of appearance: Rebellious and talented Andie (Briana Evigan) is part of a step-dance crew, but her aunt is tired of her rule-breaking and gives her one last chance. Andie can either enroll in the prissy Maryland School of the Arts or be sent to Texas. But the uptight school has no room for “real” dancing, and her old crew expels her for leaving them behind. Cue the hunky new dance partner, a new crew made up of art-school rejects, a moment when all is lost, and then an inspirational speech and a victory at the big dance competition.

Director John M. Chu is so bored that he can barely rouse himself to service his own formulaic plot, and when there’s no dancing onscreen, the movie looks like a Crate student film with shadows obscuring faces during dialogue scenes. Ms. Evigan is not a good actor, nor does she possess marquee beauty, so you assume she’s a great dancer. You assume wrong. That’s okay, though, because the real heroes of this film are Nadine “Hi Hat” Ruffin and Dave Scott, who handle the choreography. Mr. Chu is smart enough to shoot the dance numbers in long, full-body shots so you can see what’s happening, and what’s happening is some of the craziest moves you’ll ever see. They may not be your movie stars of the future, but when these dancers hit the floor and show that they aren’t to be underestimated, it’s the kind of real celebration of real dancers with real skills that makes you want to step up 2 the streets and stomp the yard because you got served.