Endearing if slight, "Superstar" at least knows what it's doing the whole way, which is more than can be said for many movies lately and most movies derived from "Saturday Night Live" skits ever.

Catholic school girl Mary Katherine Gallagher is the poster child of the walking wounded. The poor thing, horse-faced and -hipped, as yet unkissed and unloved, lethally self-loathing and yet achingly swollen with yearnings, has been a staple of the show since 1995, under the stewardship of Molly Shannon. Now as it happens, Shannon, a mature and even voluptuous woman, looks about as much like 17 as I do like 25. But realism was never and still isn't the point: The character works because Shannon so precisely gets the dank weirdness of the truly alone. And also because she shows her underpants a lot.

I do not mean that in the erotic sense. Rather, it's Shannon's absolutism, the way she breaks down all barriers between the self and the character and when, in a whirling dance of Mary Katherine's thwarted eros and selfhood, she loses all control and crashes like a fallen gymnast amid the folded chairs, her legs askew, her too-tiny schoolgirl kilt aflutter and the little white delta of her panties flapping before the world, we sense someone profoundly disturbed yet poignant at the same time.

Similarly, the movie understands the fine line between Mary Katherine's neediness, which attracts us, and her repulsiveness, which doesn't. It keeps her in this odd zone as a kind of sacred monster and it keeps her in a universe where sacred monstrosity is appropriate. It's set in an extended-sketch universe of primary colors, unlikely occurrences, cartoony opposites and complete improbabilities. The movie isn't like a sketch, it is a sketch, though an 82-minute one.

Thus its sensibility is never really conventionally narrative or even very coherent; it's anarchistic, surrealist, set in an upscale parochial school called St. Monica's that no Catholic school graduate will recognize. It's a fabulist environment in which Jesus can be a New Agey hipster who hopes to win Mary over with the Hi-I'm-Jason sincerity of a La Jolla waiter; Sky (Will Ferrell, who also plays Jesus) can be a self-satisfied parody of '50s narcissism, whom Mary Katherine adores even as we see he's not worthy; Evian (Elaine Hendrix) will be Mary Katherine's nemesis because she has a cute butt, blond hair and those eyebrows that are kind of hinged in the middle, giving her a look of wanton cruelty. The fact that all these people seem about 35 skews the thing even further into craziness.

The plot is a whisper of a murmur of a twitch, not meant to be taken seriously. When it isn't bothering to parody Brian De Palma's "Carrie," it follows as Mary Katherine, total outsider relegated to special ed (with other unusual students, like a kid who pretends to be high), aches to enter the Catholic Teen Magazine "Let's Fight Venereal Disease" Talent Contest, which, if she wins, will make her a superstar and ultimately get her kissed, preferably by Sky (meanwhile, she practices on trees and lampposts).

Of course her overprotective grandmother (Glynis Johns) won't permit it, hoping to spare Mary the fate of her parents, who were stomped to death by Irish step dancers. And the cool kids at school think it's gross that such an armpit-sniffing crazo of Mary Katherine's addled intensity should even consider such a thing. The administrators advise Ritalin. But Mary Katherine soldiers onward like a Christian soldier.

The story is basically a pretext for inspired riffs and numbers-so-bad-they're-good and for Mary's signature ritual of self-deconstruction, taking the immediate environment with her. And the fact that Shannon, a gifted comedian, isn't much of a dancer and an even worse singer, just gins the sense of the absurd up to a higher pitch.

Mary Katherine will never be confused with Madame Bovary or the French Lieutenant's Woman, but when she's disrupting the cosmos with her insistence that it notice her, she expresses a universal attribute that proclaims simply: I exist! The cosmos, of course, answers: Huh?

Superstar (82 minutes, at area theaters) is rated PG-13 for surrealistic comedy, sexual innuendo and underpants.

CAPTION: Crash courses: Molly Shannon as klutzy, pit-sniffing Mary Katherine Gallagher in "Superstar."