''But I'm a Cheerleader,'' with its crushingly obvious idea -- a cheerleader with vaguely gay longings is sent off to a heterosexual rehab camp -- belongs to that growing category of film best described as ''It Would Have Made a Great Sketch on 'Saturday Night Live' or 'Mad TV.' '' On television, where the guest host would be chewing his mouth raw trying not to burst out laughing, it would at least be short, like Robert Smigel's ''Ambiguously Gay Duo'' superhero parody, which began on Dana Carvey's short-lived show before moving to ''Saturday Night Live.'' The movie's one joke is apparent very early on, and even if you're only half watching, you'll be a step ahead.

So campy it reflexively sends an elbow to its own ribs, ''Cheerleader'' has a sweet heroine, Megan (Natasha Lyonne), who is on her high school's cheerleading squad and dates a football player. Whenever they make out, she's unengaged by his kisses; she keeps her eyes wide open, and her gaze wanders over to the other cheerleaders as her tongue absently dances with his.

''Cheerleader'' lingers over the other girls' crotches in its opening credit sequence, so Megan's attraction is suggested before the picture starts. Her parents and friends stage an intervention and ship her off to True Directions, the hetero camp run by the tight-jawed Mary (Cathy Moriarty), dressed in form-fitting pink business suit castoffs from the Mary Kay collection.

The film's leaden jokiness is magnified by having in the cast actors who telegraph that they are in on the joke. Megan's conservative, religious parents are played by Mink Stole and Bud Cort. RuPaul Charles -- yes, that RuPaul -- in a tight T-shirt and shorts you'd expect to see on a dancer in a Joey Heatherton revue, is the camp's male soi-disant deprogrammer: ''I'm an ex-gay,'' he chirps.

At True Directions, the girls' dormitory and outfits are pink. And the boys' are baby blue, as are their matching shirt, necktie and shorts ensembles, which look like the kind of thing Regis Philbin might dress his kids in. The trembling boys, who are struggling with being reconditioned, are nothing but stereotypes. And Ms. Moriarty's portrayal of the no-nonsense Mary is so butch that she's capable of emasculating any man within a five-mile radius. That would be the only possible reason her son (Eddie Cibrian) is given to wearing tank tops and Daisy Dukes cutoffs and prancing around as if he's waiting for his Village People audition. (He's also named Rock, as if this movie needed any more in-jokes.)

The girls come off better. Ms. Lyonne doesn't camp it up; she settles in and, using her spry optimism and little girl's croak of a voice, delivers a real performance. Megan is truly surprised when all the evidence of her gayitude -- a Melissa Etheridge poster, vegetarianism -- is trotted out before her. Clea DuVall, who plays Graham, the requisite tough girl and troublemaker, is reminiscent of the young Ms. Moriarty in ''Raging Bull.'' When Graham runs her hands though her hair and flirts with Megan, it's only a matter of time before they tumble. (Their love scenes are played tenderly -- the one area where the movie takes itself seriously.)