OutKast may have brought something fresh to the world of hip-hop, but their contribution to the world of movie musicals is stale. “Idlewild,” their quirky Depression-era story set to modern music, has an intriguing concept, but it’s executed so dully you’d never know it.

‘Tis a shame, for the OutKast boys — Andre “3000” Benjamin and Antwan A. “Big Boi” Patton — are personable fellows, instantly likable as screen luminaries and very talented in the recording studio. The musical numbers in “Idlewild” are lively, with amazing dance choreography and spot-on performances. So what’s the problem? Not enough of those musical numbers.

You wouldn’t think a musical would suffer from a lack of music, but here we are. “Idlewild” runs two hours and has maybe five songs. That scarcity is made worse by the flatness of what occurs in between, as the plot is cliché-ridden and uninspired. Of course, a lot of musicals come up lacking in the plot department — but they compensate for it by dazzling viewers with a lot of, you know, MUSIC. I don’t know why that thought didn’t occur to Bryan Barber when he wrote and directed “Idlewild.” He’s a music-video director, for crying out loud, yet music seems to be the last thing on his mind.

The setting is 1935, the place is Idlewild, Ga., where Percival (Benjamin), our narrator, tells us of his life-long friendship with Rooster (Patton). Percival is a pianist, the son of a stern mortician (Ben Vereen), while Rooster is a lover of gambling and booze, born into a family of bootleggers. Perhaps ironically, it is the freewheeling Rooster who is now married with children, though he offsets his lack of freedom by continuing to sleep with other women.

Percy and Rooster perform at Church, a swingin’ speakeasy-slash-brothel reminiscent of the one in “Moulin Rouge.” (The “Moulin Rouge” similarities do not end there, either, what with the sensitive narrator and the doomed romance and suchlike.) A loud fat man named Ace (Faizon Love) owns Church, but when he is killed by a ruthless gangster called Trumpy (Terrence Howard), responsibility for Church — including its outstanding debts — falls on Rooster.

Meanwhile, a new singer has arrived in Idlewild for a multi-week engagement at Church. Her name is Angel Davenport (Paula Patton), and Percy is instantly smitten with her. Their romance is addressed in tandem with Rooster’s more dire problems, though it’s done in such a way as to make the film seem unfocused. It’s never clear whose story we’re supposed to be following, Percy’s or Rooster’s

You will note that the story elements don’t sound particularly intriguing on their own, and perhaps you are already questioning the ability of a music-video director and a hip-hop duo to enact them believably. You are right to be skeptical. Benjamin and Patton are likable, as I said, but they don’t have the acting chops necessary to carry a drama like this. And while Barber shoots with pizzazz, the pedestrian dialogue he has written does nothing to spice up the bland storyline.

Those musical numbers, though, those are smokin’. Catchy, rhythmic and infectious, they hint at how great the film could have been if it had more of them, and if the plot had been boiled down to something less weighty and more enjoyable. As it stands, we are left with the mediocre remnants of a squandered opportunity.

C (1 hr., 58 min.; R, a lot of profanity, some strong sexuality, a fair amount of nudity, brief strong violence.)