The prepubescent girls who dote on the popular Disney TV series will undoubtedly embrace this big-screen spinoff, but “The Lizzie McGuire Movie” is actually little more than an adolescent wish-fulfillment fantasy so familiar that it’s like pre-chewed bubble gum. Part travelogue, part sitcom family farce, part modern Cinderella story (though one in which the handsome prince turns into a frog), part “The Parent Trap,” and part John Hughesy tale of a girl who discovers that the right fellow has been standing beside her all the time, the picture is handsomely mounted but too lightweight and by-the-numbers to do much more than take up space for a few weeks in theatres before settling into its rightful place on the VHS-DVD shelves of young girls everywhere.

Lizzie (Hilary Duff) is a junior high student with a tendency to get all flustered and embarrass herself with public pratfalls. (Perfect example: the mess she makes of her graduation ceremony.) This mini-Lucy Ricardo (who boasts a similarly scratchy, high-pitched voice) has the obligatory loving but clueless parents (Hallie Todd and Robert Carradine) and obnoxious kid brother (Jake Thomas), as well as a loyal best friend, Gordo (Adam Lamberg) and catty rival, Kate (Ashlie Brillault). Along with Gordo, Kate and doofus classmate Ethan (Clayton Snyder), Lizzie is off on a graduation trip to Rome under the harsh guidance of Miss Ungermeyer (Alex Borstein), a female version of the principal Jeffrey Jones played in “Ferris Bueller.” There she meets pop star Paolo (Yani Gellman), who’s shocked to observe that she’s an exact double for his estranged singing partner (and supposed girlfriend) Isabella (also played by Duff). The smooth Paolo romances Lizzie in a secretive, “Three Coins in the Fountain” sort of way and persuades her to impersonate Isabella in a joint appearance on a televised award show. But Gordo’s convinced that Paolo isn’t everything he seems, and a big, vaguely feminist finale that alludes to the Milli Vanilli scandal but allows Lizzie to prove her self-confidence and belt out tunes with the best of them proves him right.

With its excellent cinematography by Jerzy Zielinski, which makes good use of the Italian locations, “Lizzie McGuire” looks fine, and the supporting cast goes through the motions well enough under Jim Fall’s sprightly direction. But the whole identical double business is a hoary cliche that smells of screenplay desperation, and the device of inserting little animated segments into the live action to show Lizzie’s inner thoughts wears out its welcome well before it’s abandoned. The other problem, frankly, is Duff. She seems a likable girl, but tends to go off into exaggeration when a more restrained approach would be better suited to the big screen.

Among recent flicks targeting this branch of the youthful audience, “Lizzie McGuire” is certainly preferable to “What a Girl Wants.” But while a harmless bit of fluff, it also seems the sort of thing that, despite its length, would be more at home on the tube than in the multiplex.