Nerdy, reserved bookworm Anita "Needy" Lesnicki (Amanda Seyfried) and arrogant, conceited flag girl Jennifer Check (Megan Fox) have been best friends "since the sandbox", although they share little in common. One night, Jennifer drags Needy to a local dive bar to attend a concert by indie rock band Low Shoulder, with eyes on its blithely evil lead singer, Nikolai Wolf (Adam Brody). While flirting with Wolf, Jennifer insinuates that she is a virgin. A disastrous fire destroys the bar, and Jennifer ends up leaving the bar with the band, who take her into the woods, where she learns that the band is worshiping Satan, and that Wolf intends to use her as a virgin sacrifice in order to gain fame. However, as Jennifer was not a virgin, the demon spirit takes over her body. Later that evening, Jennifer appears at Needy's house, covered in blood, looking vacant, and vomits a trail of "black ferromagnetic fluid".[4] The following morning, Jennifer appears fine, and the small town is devastated by the destruction of the local bar. A month after the incident Jennifer is beginning to look pale and tired. That day, Colin (Kyle Gallner) asks her on a date and that night she kills him brutally while Needy and her boyfriend Chip have sex. Needy senses this and leaves Chip's house in a panic. While driving she almost runs over Jennifer covered in someone else's blood. Needy rushes home and goes to bed. There she finds Jennifer who explains what happened that night after she left the bar with the band. She even goes to show Needy what happens when she is "full" and stabs herself and the cut heals almost immediately. Needy then kicks her out of the house and Jennifer jumps out of the second story window and vanishes. The next day at school the news of a death of yet another classmate stuns the school. Needy goes to the library into the occult section to do research and finds that Jennifer is weakest when she is hungry and must eat flesh in order to sustain her life and appearance. Needy surmises that Jennifer is eating the bodies of boys to survive.


Needy decides she must stop Jennifer as she fears her boyfriend, Chip (Johnny Simmons), may be the next one she goes after. All the while the band that performed at the bar have become local heroes as they claim to have helped people out of the fire when they were no help at all. They rocket to fame and their single "Through the Trees" is the town's new anthem and the theme of the school's spring formal. Needy tells Chip not to attend the dance and breaks up with him because she fears for him but he goes against her and decides to go to the dance anyway. As he is walking to the dance Jennifer sneaks up on him, claiming she has always had feelings for him and that Needy was cheating on him with Colin. While he is upset she makes a move and they begin to kiss. Needy senses them kissing and runs out of the dance to find him. When she approaches the spot in the park where they were kissing she hears Chip yell coming from the abandoned pool on top pf a hill. As she forces her way inside she finds Jennifer feeding on Chip's neck. Then, Needy jumps in the pool and attempts to drown Jennifer but Jennifer begins hovering above the water as Needy helps Chip, who is bleeding from the neck, out of the pool. Jennifer and Needy begin to fight and as soon as Jennifer tries to attack her Chip stabs Jennifer with a pool skimmer. Jennifer bleeds a little but then escapes. Needy cradles Chip as he dies, both proclaiming their love for one another. Later, Needy attacks Jennifer in her own home, yelling, "You killed my boyfriend!" As they fight, Needy pulls out a boxcutter and cuts Jennifer twice across her stomach, which doesn't affect her. Needy rips off Jennifer's BFF necklace, which stuns Jennifer who begins to fall down to her bed. Needy comes down with the boxcutter stabbing Jennifer in the heart, and sets Jennifer free. After Jennifer dies, her mother comes and finds Needy on top of Jennifer's bloody body.


Needy is committed to an asylum for the criminally insane. Since she was bitten by Jennifer who was a demon and survived she obtained some of Jennifer's supernatural traits. With these powers, Needy escapes the facility and tracks down Low Shoulder, brutally killing them for turning Jennifer into a demon and causing all the ensuing death and destruction.