Occasionally cute but consistently dim-witted, the romantic comedy “27 Dresses” suffers the same number of generic clichés as it glides down the aisle toward a resolution as predictable as a wedding band’s set list.

But don’t blame the leads. Last year’s breakout charmers Katherine Heigl (“Knocked Up”) and James Marsden (“Enchanted”) almost salvage this shabby, flabby date movie. He displays impressive comedic timing, and she shows off her deep reservoir of charm.

While working the wedding beat for a fictional New York newspaper, Kevin (Marsden) crosses paths with perennial bridesmaid Jane (Heigl), who has stood beside 27 brides as she waits patiently for Mr. Right. Jane loves George (Ed Burns), her boss, but can’t work up the courage to tell him. The three legs of a classic love triangle begin to take shape.

So far, so good. Here comes the bad. Jane’s younger sister, Tess (Malin Akerman), blows into town. She’s introduced to George by Jane at a company gathering (believable), and they immediately hit it off (still possible). Days later, the two are madly in love (improbable, but I’ll bite). Weeks later, they’re engaged to be married (impossible). And Tess asks Jane to be her maid of honor (laughable).

Advertisements trumpet that “27 Dresses” comes from the screenwriter of “The Devil Wears Prada,” which is accurate but only half the story. Aline Brosh McKenna did adapt “Prada,” but also penned ignorable tripe like “Laws of Attraction” with Pierce Brosnan and Julianne Moore, or “Three to Tango” with television castoffs Matthew Perry and Neve Campbell. But if you put that on the “Dresses” poster, no one would show.