The film begins in a school playground, where two teenagers, Rickie (Shiloh Fernandez
) and JT (Noah Segan
) are talking. JT gets Rickie to skip school, and the two decide to go to an abandoned mental asylum. They horse around for a while, before exploring the basement. In the basement, JT disappears and scares Rickie, before they hear a banging sound. JT says it isn't him, and a large dog appears and chases them. They escape and enter another room that they must pry a rusted shut door open to enter. There they see a nude girl (Jenny Spain
) chained to a table with a plastic bag over her head. Their first impression is that there is a killer in the room or near them, but then they see her move. JT thinks that they should keep her for themselves and Rickie is horrified and wants to call the police, but JT tells him not to and punches him and Rickie leaves.
Rickie comes back later with a gun, and JT tells him that after he left the girl has started struggling, but especially that she had been gnashing her teeth and repeatedly trying to bite him. He tells him that started hitting the girl to stop her (though she posed no threat because she was chained) and that he had strangled her to death twice. He then swipes the gun from Rickie and shoots her three times, but she doesn't die. JT concludes that she is dead and wants to keep her as a sex slave. Rickie, who had initially wanted to free her is unsure how to proceed, though he wants to do the right thing.
He goes to school the next day and stares at Joann (Candice Accola
), the girl he has a crush on. Later, he talks to his friend Wheeler (Eric Podnar
) about liking Joann. He then goes to a drug dealer and purchases some bolt cutters from him. He returns to the basement to free the girl and hears a banging sound, where he finds Wheeler raping her. He argues with JT about this, and JT tells him that Wheeler is their friend and they can trust him.
He decides to go back another day, when JT isn't there and manages to cut one of her handcuffs off. JT climbs on top of her and she scratches his face deeply. Rickie gets out and goes back home.
The next day at school, Rickie talks to Joann during a gym lesson. In the car park, he and Wheeler are attacked by Johnny (Andrew DiPalma
), Joann's boyfriend and his friend Dwyer (Nolan Gerard Funk
). After being beaten up, Wheeler tells them that he and Rickie don't need Joann because they have their own pussy now. Johnny and Dwyer go to the basement, where JT asks them if they want a go on the girl. The jocks believe that she is alive and are willing to accept the implausible story that she is into being an s&m sex slave.
Having already been beaten viciously by the jocks and kidnapped by them, Rickie suggests that Johnny have the girl blow him (while his friend is already raping her) knowing that the girl will bite his penis. The wound is not shown, but it is implied to be painful but not (under normal circumstances) damaging or fatal. JT tells them that they can't go to the police about it because they will be arrested for rape.
The next day, at school, Johnny goes to the toilet and his intestines begin to fall out. Dwyer returns to the asylum basement to take the dead girl to the hospital (thinking that Johnny is diseased and not knowing that he is dead) so that they can diagnose her and cure his friend. Instead he encounters JT who kills him. There are growing reasons to believe that JT may be infected as well, potentially from the scratches to his face. Later, JT is putting makeup on the girl's face, when the dog from earlier enters the room and jumps on her, causing her to bite it and kill it.
Later, JT concludes that if Johnny was infected by the bite that they can make a new dead girl, as the original one is becoming increasingly damaged from their abuse, and they go in search of a woman. They find a woman on the car park of a petrol station, and JT hits her with a crowbar, but she manages to beat them both up and escapes. Shortly after this, Joann sees them and argues with them about what happened to Johnny, so they kidnap her instead.
They take her to the basement and tie her up, but Rickie arrives, and argues with the others, cutting Wheeler's hand off with a sword. He frees Joann, but the dead girl escapes and kills Wheeler. JT tries to stop her but she attacks him too. Rickie and Joann are trapped in the building because the exit door won't open, and Rickie decides to go back. The dead girl knocks him over rather than attacking him and pries the exit door open and escapes to freedom. Rickie wakes up to find Joann mortally wounded and stumbling towards him, saying that JT had stabbed her with the sword. JT tells Rickie to let him bite her before she dies and it's too late.
The film then shows Rickie in class with a bright outlook on life, and cycling around, before showing Joann tied to the table in the basement, apparently the new deadgirl. Rickie has nice clothes that he clearly dresses her in, but she is clearly non-verbal and an undead sex slave and the situation is no different than when JT was putting makeup on the original dead girl.