Needy (Amanda Seyfried), a nerdy and reserved bookworm, and Jennifer (Megan Fox), an arrogant and shallow cheerleader, seem like an unlikely pair, but the two are good friends. After a disastrous fire at a local bar, however, their differences grow even larger as Jennifer now has an appetite for human blood. Needy soon realizes that the string of attacks on her male classmates have been committed by Jennifer, and she is now worried that her boyfriend Chip (Johnny Simmons) might be Jennifer's next victim. Will Needy discover Jennifer's secret, and will she be able to find a way to stop her before she kills again?