Even in the brainless universe of romantic comedy, “27 Dresses” goes beyond the bounds of permissible absurdity even while it’s following every step of obligatory formula—a difficult combination to pull off. The premise is that New Yorker Jane Nichols (Katherine Heigl) is so in love with the very idea of weddings that she spends most of her time as a bridesmaid, usually the maid of honor, helping each prospective couple to plan the ceremony and pull it off perfectly, and collecting a closetful of hideous gowns in the process. The rest of her life revolves around her job as chief assistant to outdoorsman guru George (Edward Burns), a blandly amiable fellow whom she’s loved from not-so-afar for years though he remains blissfully oblivious to her stream of soulful glances. Naturally Jane has a compulsively wisecracking best friend, Casey (Judy Greer), who urges her to become more assertive in leading George to the altar.

But such is not to be, of course. Jane’s ditzy younger sister Tess (Malin Akerman), a blonde bombshell, drops in after her latest modeling gig and immediately catches George’s eye, earning Jane’s ire by lying to the guy to make him believe they’re totally compatible. Meanwhile on a night when she’s criss-crossing between two weddings Jane’s noticed by Kevin (James Marsden), the fellow whose sappy newspaper columns on nuptials she adores. They meet bad, of course—she doesn’t recognize him and is appalled by his cynicism about weddings—but he sees her wedding addiction as a good story. He finds an equally promising one in the wedding of Tess and George, for which—gasp!—Tess not only asks her to serve as maid of honor but to make all the arrangements. Matters get even harder for poor Jane when Tess announces that she wants the boathouse ceremony Jane’s always dreamed of, and will wear their dead mother’s wedding dress, which Jane had always hoped would be hers. But Kevin secretly aims to make his stories an expose of the excesses of the marriage business even as his feelings for Jane grow stronger and stronger.

You don’t have to be a genius to predict how all this is going to turn out. You know that Jane’s going to have to face her crisis of conscience over George and Tess’ impending wedding. You know that Jane and Kevin are going to bond; you even know that they’re going to have some big moment when their feelings break through—and that it’s likely to be a musical number set to an old song (here Elton John’s “Benny and the Jets,” with a gaggle of supportive onlookers). And you also know that Kevin’s stories will appear at the most inopportune time, threatening their happiness. You also know that the threat will pass, that love will overcome all, and that everything will end up happily. After all, as Oscar Wilde said, that’s what fiction means.

Anne Fletcher directs “27 Dresses” efficiently enough, but Aline Brosh McKenna’s script hasn’t a moment in it that isn’t totally unreal and actually very dumb, and it asks way too much of its likable stars. Heigl scrunches up her face and mugs for the camera to excess, desperately trying to make a character who’s actually pretty much a dunce lovable; and Marston ladles on the charm with a trowel, flashing that toothy smile so persistently that eventually you might get sick of seeing it. Akerman similarly overdoes the part ditzy, part bitchy self-absorbed sister. By contrast, Burns is so colorless that his mind appears to be on something else throughout. Maybe he’s scribbling his new “personal” script during the off-camera interludes. As for Greer, she does the Thelma Ritter, Eve Arden routine decently enough, but she can’t make Casey anything other than a walking cliché.

There’s an audience out there for this kind of sappily sweet, totally phony “falling in love” movie—and you know who you are, girls. As for you guys, be ready to head for the hills if your significant other steers you toward a theatre where it’s playing.