With title stars who'd never acted a lick and a suspicious lack of advance screenings, From Justin to Kelly didn't look promising.

Besides, it stems from reality-TV's American Idol, and the reality bent fizzled on film for April's similarly beach-set The Real Cancun.

Yet From Justin to Kelly isn't about reality. It's about the entertaining nonreality of traditional movie musicals, for which it's the first entry since Chicago revived the genre. While it's no Chicago, it is another breath of fresh air considering the long dearth of films with people bursting into song and dance.

Unlike Cancun's spring-break realities of drunken revelry and indiscriminate sex, From Justin to Kelly is a throwback to the innocent, carefree beach-party movies of the '60s. Its kids talk about hedonism, but hardly anyone acts on it, especially Justin (2002 American Idol runner-up Justin Guarini) and Kelly (2002 Idol winner Kelly Clarkson).

He's an alleged Lothario who hits Miami with pals Brandon (Greg Siff), a hustler who's sex-hungry but harmless, and Eddie (Brian Dietzen), a computer geek who even dresses like a '60s beach nerd.

Kelly is a skeptical, self-protective Texan who arrives with friends Kaya (Anika Noni Rose), the "smart girl," and Alexa (Katherine Bailess), the "party girl."

For a film that rebukes instant sex, From Justin to Kelly has no problem with instant attraction. When our heroes cute-meet in a musical frolic on the sands, they're quickly smitten, propelling the feeble plot.

Its dramatic tension comes from a wearisome string of misunderstandings, all set in motion by selfish Alexa, who sees a sea of eager guys but only wants Justin. Frankie Avalon and Annette Funicello had similar stumbles in the '60s, enabling them to skirt any semblance of carnality.

But forget sex, sin and surf. From Justin to Kelly is all about earnest boy-girl courtship and buoyant musicality.

Clarkson and Guarini are in fine vocal form as the kind of pop posers and bombastic belters that Idol celebrates. With the advantage of studio polishing, they sound better than they did on television, where they had to sing live. Their co-stars also get brief musical bits and perform quite well.

Overwrought power ballads teeter toward melodramatic mush, especially a static duet sung on a banal boat ride. But most of the largely original songs are tuneful and infectiously exuberant.

That's largely due to the real star of this film: choreographer Travis Payne. His intricate large-cast moves are geared to modern hip-hop yet also evoke the sunny, boisterous spirit of Grease. Director Robert Iscove (She's All That) also shows the right light touch.

Though screenwriter Kim Fuller's story stalls, her dialogue has some comic kick, including digs at Guarini's curly globe of hair. "I'm from Texas - I've seen bigger," Clarkson says. Another party animal calls him "Sideshow Bob."

The acting isn't fabulous, but with so much music and so little story, it doesn't need to be. By playing closely to their friendly personas, Clarkson and Guarini at least haven't embarrassed themselves on screen, which is more than Madonna or Mariah Carey can truthfully say.

Relax, Idol fans. The reality of this film is that it's much better than expected.

'From Justin to Kelly' Starring: Kelly Clarkson, Justin Guarini Director: Robert Iscove Screenwriter: Kim Fuller Now showing: at area theaters Rated PG: for thematic elements, sensuality and brief language Running time: 82 minutes Grade: B-