"Jennifer's Body" is the perfect movie at the perfect time for Megan Fox. She has become a huge star without playing a single lead role or having to really act at all (something even she admits). She has built a career on rocking red carpets and lending her eye candy to action movies.

Now she gets to prove she can carry her own movie in a horror-comedy star vehicle ideally suited for her, because the scenario for "Jennifer's Body" gives Fox a chance to make her man-eater status literal.

Gorgeous, superficial cheerleader Jennifer (Fox) and plain, smart girl Needy (Amanda Seyfried) have been best friends since girlhood. Jennifer convinces Needy to leave her boyfriend Chip (Johnny Simmons, who is awful) home one night to check out a band at a local bar.

The girls live in a perpetually boring small town, and Jennifer is so hungry for excitement that after the show she ignores Needy's warnings and piles into a van with the band.

Hours later, Jennifer shows up at Needy's house. She's covered in blood, she pulls a whole chicken out of the refrigerator and begins to eat while crouching like an animal, and at one moment she comes very close to taking a bite out of Needy's neck.

Jennifer has become (the script isn't very clear on this) some sort of demon who feeds on boys. And by feed, I mean her jaw expands like a snake with rows of jagged teeth and she tears into them, eating flesh and drinking blood.

Director Karyn Kusama borrows imagery from David Cronenberg and a satirical sense of humor from "Heathers" but molds the film into a unique experience for the first hour. Then the script, penned by Diablo Cody, the Oscar-winning writer of "Juno," falls apart completely.

"Jennifer's Body" has one of the worst endings of the year. Scratch that: it has three of the worst endings of the year. The story culminates in a showdown (this shouldn't be a spoiler - we all know how horror movies end) in what should have been the final scene of the film. Just when the writing should pack its biggest bite, the dialogue makes us cringe. This is easily the worst scene in either of the two Cody-scripted films.

Then the movie plods along through two more tacked-on, unnecessary endings, ruining what had been an entertaining, intentionally schlocky movie.

So does Fox prove she can act? Sort of. This ain't exactly Shakespeare, and Fox ain't exactly Meryl Streep.

One thing no one can deny, though, the girl knows how to use her looks. And really that's what "Jennifer's Body" is all about: the body of the high school beauty queen.

When we meet Jennifer, she exudes a bitchy yet irresistible confidence. She understands her sex appeal and uses it ruthlessly.

When she becomes the vampire-succubus-demon, blood and bile conceal the beauty, making her appear convincingly evil.

After each invigorating "feeding," she emanates the power of a glorious bird of prey. As the feeding wears off, though, she gets uglier and uglier, as if we are seeing the hottest girl in school without her make-up.

Fox steps fearlessly into each of Jennifer's states. She doesn't shy away from becoming physically ugly, nor does she mind exposing the ugliness beneath the façade of the egotistical beauty queen - no small thing, since that's how many regard Fox herself.

But surprisingly, this movie doesn't belong exclusively to Fox. It's just as much Amanda Seyfried's movie. In fact, Seyfried might pull off the bigger feat. She plays a brainy ugly duckling opposite Fox, yet she manages to steal much of the film. Not bad.

Kusama ("Girlfight," "Æon Flux") likes to play around with our notions of femininity, and there are certainly some interesting ideas and imagery floating around in "Jennifer's Body." Ironically, it's the script written by an Oscar winner that lets us down.

Jeff Marker is a media studies professor at Gainesville State College.