In his TV incarnation Patrick Dempsey may be called McDreamy, but his new romantic comedy is more like a nightmare. “Made of Honor” is basically a gender-bending version of “My Best Friend’s Wedding” in which the “Grey’s Anatomy” heartthrob takes on the Julia Roberts role, playing the long-time buddy of an about-to-be-wed woman who suddenly realizes that he’s actually in love with the imminent bride and aims to break up the coming nuptials. As the title indicates, though, in this case Hannah (Michelle Monaghan) also asks her best pal Tom (Dempsey) to serve—rather incongruously, of course—as her maid of honor when she marries a handsome, well-heeled Scottish nobleman in his ever-so-scenic homeland. That only aggravates Tom’s discomfort, of course.

This is a premise that would rankle under the best of circumstances, but under Paul Weiland’s flabby direction it plumbs the depths of the chickflick genre. As written by the trio of Adam Sztykiel, Deborah Kaplan and Harry Elfont and played by Dempsey, Tom is basically an obnoxious guy whose Lothario status is matched only by the smugness of the attitude with which he goes about bedding girls (he’s got a whole list of rules governing the process)—who, it seems, can’t help but hand him their phone numbers and beg for a second date. (It was also a mistake to begin the story with a sequence showing his cutesy first meeting with Hannah in college, in which he looks like one of the oldest frat boys in history.) And Dempsey fails to make his transformation into lovesick swain at all charming. The vapidity of the character is evidenced by the fact that, as far as I can recall, we’re not even told what Tom’s supposed to do for a living. (Maybe he’s just a jobless playboy with the predictably empty life.)

Monaghan is almost as big a problem. Hannah’s a cipher to begin with—too dense to realize Tom’s actual feelings for her until the last possible moment—but the actress does the character no favors anyway, with a performance that lacks fizz.

“Made of Honor” fumbles further by surrounding this pallid pair with secondary figures who are, if anything, even more irritating, sealing the movie’s doom with what’s often a romantic comedy’s saving grace, a great supporting cast. Hannah’s intended, Colin McMurray (Kevin McKidd) is a total stiff, and his relatives—especially his mother (Hannah Gordon) and aunt (Myra McFadyen)—are such snooty Scottish stereotypes that the characters could cause an international incident (especially when they become chief actors in a really tone-deaf final act). Sydney Pollack, who plays Tom’s much-married dad, proves a formula joke whose late scenes of self-recognition are even worse than his randy shtick early on. Even sadder is poor Selma Stern as Hannah’s grandmother, who has to do a prolonged skit involving some sex toys that’s truly excruciating. (To be fair, Kathleen Quinlan manages to maintain her dignity as Hannah’s mother.)

But it’s the pals of Tom and Hannah that really come off poorly. Her trio of bridesmaids, from bitchy Melissa (Busy Philipps) to chubby Hilary (Emily Nelson) and nondescript Stephanie (Whitney Cummings), are a tiresome bunch, and he’s got a posse (Kadeem Hardison, Chris Messina and Richmond Arquette) that couldn’t be blander. Even they’re to be preferred, though, to a gym pest called Tiny Shorts Guy (Kevin Sussman), who—one presumes—is meant to be lovably odd but comes off as really creepy instead, the picture’s worst combination of bad writing, acting and direction.

There’s some modest compensation to be found in the glossy visuals provided by Tony Pierce-Roberts, who makes the most of the exteriors in the final reels and earlier on uses the New York locations well, though one scene shot through a revolving door is a miscalculation. And overall the physical production is fine. But it’s all in the service of a misconceived idea poorly executed. Rupert Gregson-Williams’ score, though, is oppressively intrusive.

People may not recall that Dempsey had an earlier movie career marked by such bombs as “In the Mood” (aka “The Woo Woo Kid”), “Loverboy” and “Run.” You’d think that given a second bigscreen chance as a result of his success on the tube, he might choose projects more carefully. But “Made of Honor” proves that his acumen in that respect doesn’t seem to have improved with age.