It’s always been difficult for pop music stars to segue into movies; just think of the recent disastrous debuts of Mariah Carey and Lance Bass–“Glitter” and “On the Line” will forever stick in the memory (unfortunately). The makers of “Crossroads” show remarkable chutzpah by having Britney Spears start her first picture by doing a bump-and-grind number to a song from the most glaring recent example of the music stardom-to-cinematic stinkbomb trajectory: Madonna, who, with “Shanghai Surprise,” “Who’s That Girl?,” “Body of Evidence” and “The Next Best Thing,” is surely in a class of her own. Maybe the choice was a deliberate attempt to deflect criticism by subtly reminding audiences that however bad Spears’ movie might be, it pales beside the material girl’s oeuvre. But if so, it wasn’t really necessary. “Crossroads” isn’t very good, but it’s not terrible either. And Spears, who comes across as a chubbier version of Sarah Michelle Gellar, shows some promise; she’s a likable, if somewhat innocuous, screen presence.

She might be even better if Shonda Rhimes’ screenplay weren’t a sadly predictable road movie that takes Lucy, its heroine, on a route that leads from an unhappy role as a nerdy high school brainiac (quite a stretch for Ms. Spears, it must be admitted, though not one in which she’s especially convincing), through episodes of renewed friendship, exchanged confidences, young love and growing self-confidence, to a destination that involves–surprise, surprise!–a hit tune and a big recording contract. Bits and pieces of it recall the formula of “Glitter” and “Coyote Ugly” (and there are shards of “Now and Then” and “The Breakfast Club” floating around, too), and the result is a patchwork job that exhibits virtually no invention or imagination.

Still, it’s a breezy enough combination of fluff and soap opera, and should satisfy Spears’ youthful fans. In the hackneyed scenario, we meet Lucy as she graduates from high school as class valedictorian. But she’s unhappy. She’s a kid driven by her over-protective father (Dan Aykroyd)–a mechanic all too ambitious for his daughter’s success–to excel in studies, and so she’s missed out on many of the joys of adolescence; she also yearns to reconnect with the mother who abandoned her fifteen years earlier. As it happens, graduation night also requires her to get together with two childhood friends from whom she’s grown estranged: Kit (Zoe Saldama), now a snooty campus princess, and Mimi (Taryn Manning), a trailer-trash tomboy who’s gotten pregnant; they dig up a “time capsule” they’d buried years earlier and promised to retrieve after the big dance. Soon afterward the unlikely trio join up for a cross-country trip to California (their varied motives are entirely too silly to get into); they hitch a ride with Ben (Anson Mount), a hunky, laconic guitarist who’s reportedly an ex-con. Over the course of the journey they endure a host of minor setbacks, get into spats which ultimately bring them closer together, and even help finance car repairs by winning a karaoke contest in New Orleans; and, needless to say, Lucy and Ben begin making googly eyes at one another. Periodically Lucy calls her anxious dad. Ultimately everybody winds up on the Pacific beach, where everything wraps up happily for them all.

Wish fulfillment obviously runs rampant here, but if you subtract the more cloying or melodramatic elements (Lucy’s initial homecoming tryst with her nebbishy lab partner; her off- kilter meeting with her mother, played stiffly by Kim Cattrall; a mawkish subplot involving Mimi’s pregnancy and Kit’s unfaithful boyfriend; Aykroyd’s abrupt last-minute transformation– the last shot of him beaming at his daughter wiggling her way through an audition is truly weird), most of it works reasonably well. A lot of the offhanded humor is pretty juvenile, but Spears and Mount make an appealing couple, and their scenes together are convincingly affectionate. Their relationship, in fact, is the best thing in the movie; a pity the rest of the plotting so often shifts it from center stage. The rest of the cast, unfortunately, is overly strident all too often (with Saldana, Cattrall and Aykroyd the worst offenders). Clearly director Tamra Davis, while dealing nicely with the leads, hasn’t reined in the supporting cast strongly enough. The addition of a music video, interrupted by humorous outtakes, during the final crawls doesn’t help matters.

So the title of Britney Spears’ debut is fairly accurate, and the song she showcases in it–proclaiming that’s she’s no longer a girl but not yet a woman–seems appropriate, too; because “Crossroads” is neither good nor bad, but there’s enough promising stuff in it to suggest that if its young star takes the right direction, she could be the rare singer to make a successful transition to the screen.