Megan Fox and Diablo Cody Bury "Jennifer's Body"

Is it unfair to ask more of a movie than it apparently ever thought to offer? This is the maddening dilemma of Jennifer's Body, the sophomore slump from Juno writer Diablo Cody. The elements all seem to be in place for a sly genre subversion, especially from a writer with something to prove, though it may be that Cody has avoided that attitude on purpose. Either way she has avoided proving anything in particular, and by all appearances avoided writing more than two or three drafts of her second screenplay.

The Jennifer in question is a high-school cheerleader from rural Minnesota; the considerable body in question is that of Rockwood native Megan Fox, who surprises in her first big role that involves more than leaning slowly over a car hood. She and the plainer, more reserved Anita (Amanda Seyfried) have been close friends since childhood—or "biffs," as Jennifer says in Cody-speak as she displays a shared BFF pendant—but Jennifer's jail-baiting and domineering are slowly driving a wedge between the two.

Still, Anita (nicknamed "Needy," as if she were the one benefiting from the friendship) is willing to ditch her doting boyfriend in favor of tagging along to the local dive bar to see big-city wuss-rock band Low Shoulder, where Jennifer and the band's dreamy—sorry, "salty"—frontman (Adam Brody) make an altogether different sort of connection than intended. Needy goes home alone, only to find a blood-covered, half-crazed, black-goo-vomiting Jennifer in her kitchen in the middle of the night.

Soon after that, local boys start getting eviscerated.

The implications of Cody's scenario are obvious, and rich. The story could explore Jennifer's journey from foxdom into fiendhood in any number of ways, from the entitlement of the beautiful to a more sympathetic girl-power revenge angle. That Cody is not only one of Hollywood's highest-profile screenwriters but also one of only a few female screenwriters with any profile at all would seem to leave only the question of which approach she'd take.

Maybe there were just too many options to choose from, because the theme Cody settles on amounts to little more than "Teenage girls are monsters, am I right?" Turns out that Jennifer's Body is nearly as shallow as its subject, indifferent to big ideas and focused instead on being a perfectly run-of-the-mill teen horror movie. Despite being of clear value to a film engaging high-school archetypes, the sharp emotional intelligence that elevated Juno is nowhere to be found; this leaves most of the characters ill-defined, and the film pays a price for it whenever the narrative zig-zags and none of the characters can be bothered to react with any consistency. Worse still, the few stray whiffs of feminism are negated entirely by a clunky sapphic detour (the fruit of a purposeless subplot) that exists solely to throw the guys in the audience a bone.

The film is empty-headed, then, but not all bad. Director Karyn Kusama draws some good performances from her cast, does her best to keep the scares atmospheric, and manages to smooth over some of the script's indulgent moments. (At least Jason Reitman got to cower behind a corny-indie veneer with Juno; horror is less forgiving to a talky script.) And it must be admitted that Cody racks up a fair number of both laughs and gasps, sometimes even at once. The Low Shoulder subplot hits some fine satirical notes—their impromptu Tommy Tutone cover is the only culture-referential gag that really succeeds—and there are a handful of genuinely disturbing moments.

Isolated flashes of wit and inspiration, though, aren't enough to sustain Jennifer's Body, and their scarcity isn't its only problem. The performers occasionally trip over the undercooked rhythms of Cody's dialogue and the labored, asinine slang intended to distinguish it. Good and bad ideas alike trail off within the narrative, getting reintroduced when there's more important business afoot or just dissipating altogether. Worst of all, the bulk of the comedic and horror elements find themselves at odds with each other, leaving the film's tone a mush.

There was a damn fun horror movie to be made of Jennifer's Body, but somebody forgot to write it.