Starring Lindsay Lohan, Tina Fey, and Rachel McAdams. Rated PG.

The best-ever Saturday Night Live spinoff isn't actually a spinoff. That's because no skits were expanded (or harmed) for this detailed, deliciously cutting look at high-school Girl World, 2004 style. Instead, SNL head writer Tina Fey has fashioned a script that actually goes the whole distance to make Mean Girls funny and resonant on several levels.

She started not with the usual teen novel but with a nonfiction book called Queen Bees and Wannabes. This might sound like a nonstarter, but it allows Fey to get all anthropological on us by dissecting today's campus life along tribal lines (jocks, cool Asians, mathletes, et cetera). She was further encouraged by a Paramount executive, believe it or not, to turn her fictional, home-schooled hero into someone fresh on the scene from Africa.

The device is brilliant, lending what you might call a kind of naturalistic absurdism to the outlook of Lindsay Lohan's tomboyish Cady, who not only views the antics at her first American high school with a zoologist's eyes but also sees the kids go ape on occasion--in Animal Planet--like "nature" scenes augmented by African-themed music.

The note-perfect Lohan here reteams with Freaky Friday director Mark S. Waters, whose brother, Daniel Waters, wrote the influential Heathers. The new film's updated Heatherettes, now called the Plastics, are neither quite so lethal nor nearly as hateful in the end-- although our hero soon learns, the really hard way, just how dangerous they can be. Initially revolted by the shellacked trio led by íƒ ¼berblond Regina George (up-and-comer Rachel McAdams), Cady is encouraged to infiltrate the Plastics by so-out-they're-in "art geeks" Janis Ian (Lizzy Caplan), presumably named after the 1960s folkster, and Damian (Daniel Franzese), who is proud of being "almost too gay to function".

Regina runs a tight ship--pink on Wednesdays and no halter tops two days in a row--but her specialty is the withering putdown dressed as a compliment. Amanda Seyfried plays the platinum sidekick too opaque to be bothered by much, and Lacey Chabert, as the resident brunette, is a powder keg of anxiety, expecting to be cast out at any moment. Cady is initially fascinated by the trio's pointless power plays, but, like smaller primates, she quickly finds herself sucking up and dumbing down for the sake of survival and--worse--any hint of approval from above.

Soon enough, Cady's actual friends get tired of hearing about the fascinating horrors of Regina George, but by then there's the additional problem of falling for the queen bee's ex-boyfriend (Jonathan Bennett).

Fey herself plays the no-nonsense teacher who tries to help Cady get ahead, and the cast is sprinkled with other SNL vets, notably Tim Meadows as the school's seen-everything principal, Ana Gasteyer as Cady's clued-out mother, and Amy Poehler as Regina's surgically obsessed "cool mom". The film flags slightly when attempting to soften things near the finish line, but there's already more edge exposed than you get in most indie flicks. And, of course, it's very educational.