Mean Girls is the second big-studio comedy released in as many weeks to be engineered from 1980s teen-movie DNA: The ancestry of ''Heathers'' is evident in this sweet-tart comedy about female bitchiness in the high school jungle, just as ''Big'' leaves its genetic fingerprints on ''13 Going on 30.'' But ''Mean Girls'' is the first feature made from the stuff of Tina Fey, a head writer at ''Saturday Night Live'' and exemplary Head Girl when it comes to sexy-smart. Hers is a feminine comedic sensibility in a mixed-message age of ''The Man Show'' leers and Britney Spears wiggles, snark from The Onion and soap-bubble simpers from Jessica Simpson. And as such, the movie -- a vinegary fable with a Splenda aftertaste -- is a harbinger of hope not only for future feminist comedies of any grit but also for ''SNL''-staffed feature films that don't disproportionately suck.
''Mean Girls,'' written by Fey and directed by Mark Waters with the same gangly energy he brought to ''Freaky Friday,'' marches along for good stretches of time to the toot of its own saxophone. (If only those damn mellow violins of lessons learned didn't sough in the end.) The story builds from the sociological foundations of ''Queen Bees and Wannabes: Helping Your Daughter Survive Cliques, Gossip, Boyfriends and Other Realities of Adolescence,'' Rosalind Wiseman's sharp-edged dissection of teen hierarchies. But for marquee purposes, Wiseman's brutal thesis -- that adolescent girls are regularly cruel to one another in ways that would embarrass jackals -- has been softened into a photogenic fictional narrative suitable for in-demand teen actress (and ''Freaky Friday'' star) Lindsay Lohan, and shaped to include a clutch of ''SNL'' performers in cameos as teachers and parents. And for Fey's own pleasure, the script now includes excellent clique IDs straight out of an ''SNL'' sketch. (My favorite lunchroom table: ''unfriendly black hotties.'')
Cady Heron (Lohan) enters high school in the suburbs of Chicago fresh from Africa, where she grew up, homeschooled by her researcher parents. In other words, Cady's isolation in nature, away from the trappings of American civilization, makes her the perfect blank slate on which to record the evil that girls do. (She's a tasty bit of fresh meat, too; Cady may have grown up in the bush, but embodied by the mascaraed Lohan in cleavage-busting little tops, the child clearly wasn't deprived of a subscription to Lucky magazine.)
The new student is easily adopted by a duo of arty outsiders. The math nerds want her in their club too. But Cady is also courted, in a primate-smart way, by a trio of vixens known as the Plastics, led by Regina, the school's imperiously blond queen bee (''The Hot Chick'''s indistinct Rachel McAdams), with vacuous yes-girls Gretchen (''Party of Five'''s Lacey Chabert) and Karen (Amanda Seyfried) trailing in her wake. And egged on by her arty prankster pals, who urge her to infiltrate enemy territory as an undercover agent, Cady finds herself turning more and more into a backstabbing Plastic bitch herself. (The comedic downside of Lohan's rapid rise into a polished 17-year-old star is her increasing discomfort with playing goofy, the flaw of precocious sophistication that marred her performance in ''Confessions of a Teenage Drama Queen.'')
Cady's reversion from smart young woman to savage cat is observed by Fey, playing the sardonic math teacher Ms. Norbury. And in the movie's lesson plan of a moral climax, Norbury -- signature ''Weekend Update'' eyeglasses happily in place -- lectures the entire female student population, as well as those sitting in movie-theater seats, on the hurts they inflict so casually when they play feline games. ''Calling somebody else fat will not make you any thinner,'' she proclaims. ''Calling somebody stupid will not make you any smarter. And you've got to stop calling each other sluts and whores. It just makes it all right for guys to call you that.''
The movie is full of good ideas like that -- ideas that parents, or at least adult viewers, may appreciate more than the target audience. But it's not the speeches that matter in this activist comedy; it's the light-touch vignettes that carry the most meaning in ''Mean Girls'': Look behind the glasses to Fey's invaluable expressions of womanly deadpan dismay for advice on all things modern-girl.