Anita "Needy" Lesnicki, portrayed by Amanda Seyfried, begins her harrowing tale as a violent inmate in a mental institution, reflecting back on the events that brought her there. She and her childhood friend, the popular cheerleader Jennifer Check, played by Megan Fox, maintain an unlikely friendship despite their contrasting personalities. Their lives take a dark turn one fateful night when Jennifer invites Needy to a concert by the indie rock band Low Shoulder at a local dive bar. A catastrophic fire breaks out during the show, leading to numerous fatalities, and although Needy desperately attempts to pull Jennifer away from the band, she ultimately leaves with them.
The next morning, a traumatized Needy encounters a bloodied Jennifer in her kitchen, who horrifyingly eats food from the fridge and then vomits a mysterious black, spiny substance. When Needy tries to voice her concerns the following day, Jennifer brushes them off, all while the town mourns the loss of life. In a shocking series of events, Jennifer seduces the school's football captain, only to brutally murder him in the woods shortly thereafter. Meanwhile, the band’s reputation skyrockets due to rumors surrounding their supposed heroism during the fire, leading them to secure an invitation to the school's spring formal.
As time passes, Jennifer’s appearance begins to fade, prompting her to accept a date with goth/punk Colin, played by Kyle Gallner. That night, something within her awakens as she savagely kills him. While Needy is intimate with her boyfriend, Chip, played by Johnny Simmons, she senses a chilling disturbance, which leads her to a bloody encounter with Jennifer—the same girl she thought she knew. It is revealed that Low Shoulder had offered Jennifer as a virgin sacrifice to Satan in their desperate bid for fame. However, Jennifer, having been killed in the process, is now a demonic entity.
In a twist of fate, Needy discovers that the only way Jennifer can sustain her horrifying new life is through human flesh. After the shock of Colin’s death, Needy rushes to the library's occult section to uncover the truth about Jennifer’s transformation into a succubus. Despite warning Chip, he dismisses her fears and attends the dance, leading to a disastrous confrontation where Needy finds Jennifer feeding on him. A desperate struggle ensues, culminating in Chip's tragic demise as he tries to save Needy from the clutches of his seductive killer.
Driven by heartache and the need for vengeance, Needy confronts Jennifer at her home, where she finds her feasting on potential victims from her yearbook. A fierce battle between the two ensues, culminating in Needy plunging a box cutter into Jennifer's heart, successfully destroying the entity that had taken over her friend. As Needy is captured, she comes to realize the dark gift she inherited from her encounters with Jennifer—supernatural abilities that could fuel her thirst for revenge. After escaping from the asylum, she sets her sights on the band that initiated this nightmare, ready to unleash her fury upon them for the horrors they inflicted.
