Medical student Zoe Parker reluctantly attends a party at her medical school, Whittendale University, at the behest of her friends. While retrieving a keg of beer from the morgue, Zoe is confronted by her disturbed patient Max, who is obsessively in love with her and whose blood has an unusually high level of antibodies. He attacks and attempts to rape her, but a corpse reanimates and bites into Max's throat. Zoe flees and warns everyone at the party, but hordes of the corpses, later dubbed rotters, burst in and kill all except Zoe, who escapes through a window, only to find the city besieged by rotters.The survivors of the initial outbreak are sent to refugee camps. Zoe is sent to High Rock Emergency Bunker, run by Lieutenant Miguel Salazar.Five years later, High Rock Emergency Bunker has lost contact with the other camps and with headquarters. Zoe and her friend Elyse serve as the camp doctors, treating illnesses and using rotter blood samples to create a cure for the rotters, although Miguel believes a cure is impossible.Miguel, at Zoe's urging, sends Zoe, Elyse, his brother (Zoe's boyfriend) Baca, Frank, Lucy, Derek, and Thomas on a supply run to retrieve medication from Whittendale University for Lily, a little girl suffering bacterial pneumonia. While taking medication from a professor's office, Zoe encounters Max, now partially a rotter. Horrified, Zoe flees and accidentally attracts the attention of other rotters. The group saves her, but Frank is infected and is killed. Zoe successfully administers the medicine to Lily; however Max has managed to enter the camp.Due to Frank's death, animosity between Zoe and Miguel increases. While walking on her own, Zoe is attacked by Max, but signals for help and is rescued by Miguel, Baca, Thomas, Lucy, and Derek. Realizing that Max is not fully a rotter, Zoe convinces Miguel to allow her to use Max's blood to create a vaccine, and Miguel gives her 48 hours to create it, having been convinced by Frank's wife Elle.Zoe and Elyse realize they need to test Max's blood on blood samples from live rotters. She convinces Baca to help her by opening the gates to allow one rotter in and take a blood sample from it. Although this initially goes well, rotters burst through the gates, kill Thomas, and infect Elyse. Miguel kills several rotters and seals the doors to the compound, and, ignoring Zoe's pleas that she can use the vaccine on her, kills Elyse.Miguel tells Baca that earlier he saw Zoe's name carved into Max's arm. When Baca confronts Zoe about this, she tells him about Max's rape attempt.Max steals the keys to his handcuffs from Alphonse during a scuffle. Max begins to taunt Zoe, telling her repeatedly, "You are mine", before freeing himself and attempting to rape her again. However, he is fended off by Baca.Lily's mother, whom Max had previously infected, chases after Lily and kills Derek. Max kills her and chases Lily to the motor pool.The remaining military personnel arrive, but Max opens the doors and allows a horde of rotters inside, which kill Miguel, Elle, Lucy, and several others, although most of the rotters are killed. Lily flees outside, and Zoe chases after her.Max follows them into a greenhouse, where Zoe, hidden in mud, disembowels and decapitates him. Zoe and Lily walk back into the compound, where she finds an infected Baca about to commit suicide. Zoe convinces Baca she can cure him with the vaccine and injects him with it, curing him. Sometime later, High Rock Emergency Bunker has been repaired, and Zoe sends a message to any remaining survivors that they have the vaccine, while the rotters' growls are heard from within a nearby forest.