"Sandbox love never dies," mousy high schooler Needy Lesnicky (Amanda Seyfried) says when explaining why cheerleading bombshell Jennifer Check (Megan Fox) has remained her best friend since childhood. Maybe so, but not all BFFs willingly make out—with tongue—after one of them is suspected of demonic possession and confesses to a murdering spree. Nevertheless, that's just how the quirk-filled, metaphor-heavy "Jennifer's Body" rolls. Stylishly directed by Karyn Kusama (2005's "Aeon Flux") and written by Oscar-winning dialogue extraordinaire Diablo Cody (2007's "Juno"), this horror/comedy hybrid defies easy categorization while keeping provocatively suggestive of the inner wants and desires that everybody experiences, but do not always act upon. To be sure, Needy has a healthy relationship with adorable boyfriend Chip (Johnny Simmons), but one can sense in the glances she steals at Jennifer—and later the devotion she affords her in extreme situations—that there is more going on beneath the surface than is outright spoken. Needy grows terrified of Jennifer, all right, and not just because her more popular pal has become a boy-eating demon; Jennifer also serves to remind Needy of her own uncertain sexuality, a truth she doesn't think she's ready to face.

In the small Midwestern town of Devil's Kettle—named after a mysterious whirlpool in the rock bed of a local creek—nothing much of excitement ever happens. That all changes when a nearby bar burns to the ground, several students and a teacher among the casualties. Having come to see indie rock band Low Shoulder perform, Needy and Jennifer were there at the time and narrowly escaped. So did the band members, who become overnight heroes and celebrities, the song they were singing at the time of the blaze, "Through the Trees," adopted as the tragedy's mournful anthem. Needy, however, knows the truth—that Low Shoulder were not saviors of any sort, helping no one and whisking a shell-shocked Jennifer off in lead singer Nikolai's (Adam Brody) van even as people were still running around on fire. The next time Needy sees Jennifer is later that night, covered in blood and throwing up a strange black substance. The following morning, she acts as if nothing happened, oddly aloof to the whole ordeal. Then the bodies of male classmates start showing up, ripped apart by what looks like an animal but Needy knows is really Jennifer, in need of their flesh and blood to satiate her hunger and keep up her striking looks. As Needy tells a skeptical Chip, "Jennifer's actually evil. Not high school evil."

"Jennifer's Body" could strip away its violence and otherworldly elements and would still work as a supernaturally-tinged emblem of teenage life. It's a time when romantic breakups, lovelorn pining from afar, falling-outs with friends, and a bad grade on a test can seem like the end of the world. Jennifer would appear to have a lot of power in the hallways of Devil's Kettle High School—she's beautiful, popular (but not as popular as she once was), and has a witty retort for everything—but even she is insecure about herself. It's a fact that few know except for Needy, a comparative wallflower who comes into her own even as her state of mind slowly unravels. It is she, and only she, who is aware of Jennifer's murderous new extracurricular activities, and with no one else to help—her mother (Amy Sedaris) informs her she is not always going to be there to protect her—it is up to Needy to find a way to stop her. This is ultimately easier said than done, though, for a girl whose past strength has come from her close association with Jennifer. They are supposed to be best chums, after all, and, even if she has changed, the real Jennifer still exists somewhere inside this monster. It's that old Jennifer that Needy is having trouble saying good-bye to, and might even have stronger feelings for than she lets on.

Stripped of these underlying psychological inferences and judged on its own cinematic merits, the film is more uneven, not quite as sharply satirical or groundbreaking as its past brethren, 1989's "Heathers" and 2004's "Mean Girls." The humor comes primarily out of Diablo Cody's brilliant dialogue, so fresh and poppy that it practically invents its own vocabulary, and this is its strongest suit. Still, since a lot of the picture is intended to be taken seriously, it rarely, if ever, achieves side-splitting status. Alternately, the winking smile-and-nod moments of comedy get in the way of the horror, which is rarely, if ever, truly scary. By blending both tones but not quite getting the mixture right, "Jennifer's Body" works better, then, if viewed simply as a slice-of-life with fancifully gruesome elements. The most captivating parts are when the touchy relationship between Jennifer and Needy takes center stage; the death scenes and more typical fright moments of walking through dark houses alone or being startled when something pops up are moodily shot by cinematographer M. David Mullen (2006's "The Quiet"), but pretty standard just the same.

Current "It" Girl Megan Fox has been getting the bulk of attention for "Jennifer's Body," which, unlike in 2007's "Transformers" and 2009's "Transformers: Revenge of the Fallen," actually requires she act and put more than two words together. Lo and behold, she actually impresses, handling the dialogue with aplomb, earning some laughs from her on-target deadpan delivery, looking ominous when need be, and building a real-seeming character at the onset whose naiveté spells her undoing. The jury is still out on whether Fox will be able to believably play more diverse roles outside her comfort zone, but she does well here. As heroine Needy, Amanda Seyfried (2008's "Mamma Mia!") is the real star of the movie. It is primarily through her eyes that the story is told, and Seyfried is a natural presence worth following who never overdoes her character's nerdiness or comes off as a pariah. Instead, she is merely under-the-radar, but not wholly overlooked; at least she has a boyfriend, which is more than can be said for Jennifer.

As said boyfriend Chip, Johnny Simmons (2009's "Hotel for Dogs") plays him as irresistibly earnest, reliable and heartfelt, the guy of anyone's dreams. As Satan-worshiping Low Shoulder frontman Nikolai, Adam Brody (2007's "In the Land of Women") creepily plays against-type while wearing a pound of eyeliner. Kyle Gallner (2009's "The Haunting in Connecticut") is also memorable as goth kid Colin, who can't believe it when Jennifer suddenly expresses interest in him and starts thinking with the wrong head. By comparison, the adults on display are less developed and given little to do, though Amy Sedaris (2009's "Dance Flick") makes an effective impression in her too-brief scenes as Needy's overworked mom.

"Jennifer's Body" is smart and looks great, but is it as iconic or indelible as it strives to be? Probably not, for the road it travels has been done better—and with more insight—in the past. The wraparound scenes set in a mental institution are unnecessary—they feel tacked-on—but the end-credits montage of carnage scored to a brooding cover version of Blondie's "In the Flesh" brings things around to a satisfying conclusion. Needy, changed forever but no longer, well, needy, has been marked by Jennifer in more ways than one, and knows exactly what has to be done. Mostly fun but finally downbeat, "Jennifer's Body" runs into a spot of confusion as it tries to figure out what it wants to be. Fortunately, it's better to overcompensate than to be nothing at all.