If you’re going to make a one-joke comedy, it had better be a pretty good joke, and you’d better be imaginative in giving it twists and turns that can sustain interest over feature length. “White Chicks” fails on both counts. This new effort from the Wayans brothers–Marlon and Shawn star as well as co-writing it with older sibling Keenen Ivory, who also directs–is the year’s second “Some Like It Hot” ripoff, following “Connie and Carla,” in which two women fleeing mobsters pretended to be drag queens in a San Francisco gay club. Here Marlon and Shawn play inept FBI agents Marcus and Kevin Copeland, who pose as Tiffany and Brittany Wilson, a couple of high society East coast debutantes, in order to foil the girls’ threatened kidnapping. (Originally they were assigned by their predictably explosive boss, played by Frankie Faison, only to bring the two snooty brats to the Hamptons for surveillance, but after bungling that task they decide on their own to undertake the unlikely impersonation.) What results is a woefully unfunny slapstick farce in which even the makeup is bad.

Marlon and Shawn are supposed to be lovable goof-ups with an apparent penchant for disguise–in the terrible opening scene, they mug it up as a couple of Latino shop clerks in a failed effort to nail some drug dealers–but no fewer than six scripters (including the brothers themselves) have been able to come up with only the most obvious riffs for the plastic-laden leads once the “girls” reach their high-society destination. They naturally duel with two catty arch-rivals (Jaime King and Brittany Daniel) whose father (an especially slimy John Heard) is the master of the locality, becoming extra-popular in the process.. They also encourage three other girls (Busy Philipps, Jennifer Carpenter and Jessica Cauffiel) to gain self-confidence by being themselves, while putting the guys in the “cool crowd” in their proper place. In the obligatory smarmy tangents, the white “female” Marcus becomes the prey of a lustful black football star named Latrell (scenery-chewing Terry Crews)–a circumstance that’s not only embarrassing but raises the hackles of the overly-suspicious wife (Faune Chambers) Marcus left back home–while Kevin drops his drag act periodically to romance Denise (Rachelle Aytes), a visiting reporter. And–oh, yes–the pair eventually cracks the case in a big, splashy finale, even though they have to outwit their biggest bureau rivals (Lochlyn Munro and Eddie Velez) to do so.

This is supremely silly stuff, and it hasn’t been shaped by the six–count ‘em, six–writers for much comedic effect. There’s an undisciplined character to it all, with Keenen’s direction strangely flaccid for the most part, allowing entirely too much leeway to his younger brothers to mug through their usual shtick. As is normal in this sort of cross-dressing farce, the central imposture isn’t remotely convincing–it wasn’t possible to believe that Nia Vardalos and Toni Collette could be taken for men in “Connie and Carla,” and it’s even less so to accept that the Wayans brothers, encased in glutinous plastic suits that leave them with macabre, toothy mouths and skin the tone of Michael Jackson’s, could pass for white girls for even an instant. (To assign credit or blame where it’s due, the makeup effects were created by Greg Cannom and Keith Vanderlaan.) But the physical failings are the least of the picture’s problems. It’s the awful writing that really grates. It reaches its nadir in the last act’s required “Nobody’s perfect” moment, the attempt to match Buster Keaton’s famous final line from “Some Like It Hot,” assigned to Crews when he discovers that Marcus is a man. Here that classic Billy Wilder instant (complemented, of course, by Jack Lemmon’s wonderful reaction) is replaced by dialogue so crude and insulting that no viewer, of either gender or any racial background, will be able to watch it without feeling debased. Marlon’s double-take looks appropriately appalled, though.

Needless to say, everybody in the supporting cast suffers, as the unfortunate Crews does, from the unappetizing nature of the material. Faison and Munro probably come off worst, but the level of degradation is merely relative. The picture doesn’t look terribly attractive, either, with a rather cheesy production and photography by Steven Bernstein that’s at best functional. Teddy Castellucci’s loud, obnoxious music adds to the overall feeling of bedlam.

Undemanding viewers who chortled through the first two “Scary Movie”s might find this latest slice of Wayans mania enjoyable. But those who recall the sharp miniatures of “In Living Color” will watch the bloated “White Chicks” with regret that these talented brothers have so completely lost their old edge.