Romantic comedies are becoming an endangered species in part because they are so predictable. 27 Dresses sets up an amusing concept, then coasts on the charms of its star, Katherine Heigl. Pleasant, diverting, occasionally even funny, it fits into the shrinking "date movie" niche by hewing to strictly formulaic plotting.

Heigl plays Jane Nichols, a repressed, driven career woman who still believes in happy endings. Secretary to George (Edward Burns), an "eco-friendly, philanthropic" sporting-goods magnate, she avoids her secret infatuation with him by immersing herself in the bridal culture. Sort of an amateur wedding planner, she can effortlessly juggle all the details of ceremonies and receptions. Her reward is a collection of bridesmaid dresses she keeps stuffed in a closet, along with her emotions. But when her younger sister Tess (Malin Akerman) starts an affair with George, Jane's life threatens to become unhinged.

Complicating matters is Kevin Doyle (James Marsden), a wedding reporter who professes to hate the bridal industry, and who makes Jane's obsession the centerpiece of a mean-spirited newspaper article. Arranging a happy ending is simply a matter of lost wallets, plenty of alcohol, two betrayals, and some halfhearted sermonizing.

In spite of its smart premise, Aline Brosh McKenna's (The Devil Wears Prada, but also the appalling Laws of Attraction) script seems compiled of bits and pieces that worked better in other movies. Hidden identities, sibling rivalries, a drunken song and dance, a mean trick that backfires—all are tossed together with little regard for logic or momentum. Choreographer-turned-director Anne Fletcher adopts a peculiarly heavy, graceless tone. Even the film's centerpiece—Heigl modeling the titular dresses—comes off as long and clumsily staged.

In Hollywood's heyday, leads in a romantic comedy just had to look glamorous while character actors did all the work. But the supporting cast here—apart from Judy Greer, who does a vivid, hilarious turn as a drunken, slutty friend—is uniformly bland and forgettable. Why give so much screen time to a cab driver if he isn't going to say anything funny? Why direct Edward Burns to play even more woodenly than usual?

It's hard to blame Heigl, who is spirited and affable throughout. Her graciousness, comic timing, and willingness to make fun of herself will go over well with audiences. (After Knocked Up, she is also now the movie star who gets drunk and has sex.) Marsden, who made a strong impression as the larger-than-life prince in Enchanted, almost pulls off another too-good-to-be-true role. But asking the boyish actor to play cynical and heartbroken doesn't work. What's worse, he strikes no sparks with Heigl, appearing no more than professionally polite.

Then again, this is a movie that trains its heroine to settle for someone she doesn't like, and who doesn't like her, so a certain amount of coolness is understandable. It's a dispiriting moral for a film that aspires to fairy-tale lightness. Heigl deserves a better vehicle.