The title character in "Jennifer's Body" has a novel eating disorder. Megan Fox plays Jennifer, who gets a makeover from high school vamp to demon bloodsucker. She binges on the guts of high school boys and purges in torrents of black projectile vomit. In similar fashion, the film gobbles up horror-movie clichés and sploshes them onto the screen in half-digested disarray.

This is the kind of fiasco so jarring that you wish the ushers would wrap you in a shock blanket and hand you a cup of cocoa. Fans of Oscar-winning screenwriter Diablo Cody (and I count myself a big one) can only hope her followup to "Juno" will soon be forgotten, an awkward asterisk in a proud career.

The story is narrated by Jennifer's best friend, Needy ("Mamma Mia's" Amanda Seyfried), whom we meet as a violent inmate in an asylum. "I used to be normal," she declares. "Well, as normal as any girl under the influence of teenage hormones." Back in Devil's Kettle High School, she was the nerdy nice girl to Jennifer's cheerleader Queen of Mean. Jennifer wants a wingman for her trip to a roadhouse where a cool rock band is playing. The alpha girl wants to sink her talons into lead singer Nicolai (Adam Brody). We soon learn that he's trolling the hinterlands for a virgin sacrifice to Satan (the reason for the offering is one of the movie's few well-conceived jokes). Jennifer, however, is no virgin, and through "demonic transference" she returns to school with an insatiable thirst for boy blood.

It's a serviceable premise, but the execution fails on almost every level.

The tone wobbles like a tray of Jell-O shots. Director Karyn Kusama ("Aeon Flux") can't decide if she's making a girl-power gore movie, a lampoon of exploitation flicks, or a parody of a Diablo Cody film. Teens discuss the Jennifer situation in hyper-articulate Codyspeak ("She's really evil, not high school evil"), but fail to call the authorities to do something about it. A campy scene of Fox shredding jugulars will be followed by a solemn shot of the victim's keening parents.

If the feel that they were aiming for was grisly hilarity, with laughs providing a quick reprieve from nail-biting tension, Kusama and Cody utterly miss the target. The effect is not tongue-in-cheek, it's foot-in-mouth -- all the way up to the knee.

Kusama has zero feel for the dance of anticipation and surprise that marks a good horror movie. She tries to make kill scenes double-scary by pouring on more ketchup. Not that the poorly paced script gave her much to work with. Cody mostly shuffles stock scenes from the fright film template and stirs in her patented Dialogue Helper: "What's up, Vagisil?" "Where's it at, Monistat?"

Which brings us to the matter of Fox, the most mentally checked-out actress this side of Jessica Alba. That tinny voice. Those vacant eyes. That leer-but-don't-touch strut. Seyfried sells every scene she appears in, even a gratuitous, out-of-nowhere two-minute lip lock with her co-star. She's the true star of the show. But it's Fox's movie, her ascent to leading lady status, and she is simply cringeworthy. I've seen balloon animals with more presence. The decent thing to do with "Jennifer's Body" would be to cremate it and scatter the ashes.

Colin Covert • 612-673-7186