In a tense prologue, Alyson and Josh, a teenage couple, search for a private place to be alone, slipping into an odd abandoned structure on the outskirts of town. They come upon a moldy sofa in an otherwise empty room; [Alyson] removes her shirt, and the pair begin to initiate sex, but [Josh] is frightened by what looks to be a metal hand wrapping its way around the door frame. Fearing they are being spied on, [Josh] backs away, which annoys [Alyson], who steps into the hallway for a cigarette. She is grabbed by an unseen creature and dragged away. [Josh] goes looking for her and finds her alive but soaked in blood. He reaches out to help her, but a tall woman with sharp teeth and steel scalpels affixed to her fingers leaps on him, opening his stomach and squeezing out his liver and intestines while Alyson screams.
Meanwhile, Rachel is a college freshman invited to celebrate the completion of her first year with her five friends Louisa, Derek, Francis, Leticia, and Troy. Rachel has been childhood friends with [Louisa] for years, but while Rachel is studious and responsible, Louisa is a reputed troublemaker who carries a switchblade knife. She whips it out at Troy after he jokes about her being a lesbian, but then lets him go. Francis has a crush on Leticia and hopes to hook up with her at the party that night, picking up the gang in his van. The six teens are unsure where a good place to party would be, prompting Louisa to suggest the Dyson Chemical Factory at the edge of town, which has been abandoned since they were small children.
In the factory, a hobo, Glen, stumbles drunk into the main lobby, looking for a safe place to crash. He comes across Alyson, who warns him of Troy’s murder. Glen hugs her and tries to reassure her that she’ll be alright, but the murderous woman who killed Josh then grabs Glen and tears out his beating heart, bites it, and then attacks Alyson, killing her off-screen by biting her neck and drinking her blood. Rachel and her friends prepare for the party that same night, gathering supplies, and upon arrival, Louisa reveals the factory’s history: Dyson Chemical had severely injured a worker in an industrial accident, and the worker became ill from the effects of the unknown toxin she had been exposed to, sent home without compensation. Presumed dead, she later returned to Dyson Chemical and slaughtered each of her colleagues, after which the factory was shut down. Rachel is bothered by the story, but tries to relax at the party, hoping to spend time with Derek alone. As the gang gets drunk and high on marijuana, Francis and Leticia wander off to a room with a rotting mattress, where they engage in long-winded sex.
As the party winds down, the murderous Alexa begins attacking the teens. She stabs Troy to death, castrates Francis by squishing his testicles until they pop, and then seeks out any other survivors, causing Rachel, Derek, Leticia and Louisa to flee. Leticia is cornered by Alexa and has her eyes pressed in until she’s blinded, while the remaining three teens escape into a hollow corridor. They find that the end of the corridor is sealed up with cinder blocks, and Derek tries to defend the girls by confronting the killer, only to be knocked unconscious and stabbed in the stomach. Louisa grabs Rachel, preventing her from leaving, and reveals that the killer’s name is Alexa; Louisa was Alexa’s kid sister, and has been luring victims to the factory for her since she was a child. To Louisa’s shock, Alexa, who has no mental faculties left, attacks and kills her. Rachel is finally able to overpower Alexa by knocking her down and stabbing her with her own scalpel-accented hands. Rachel faints, later waking up in a hospital where the doctor informs her that she is the sole survivor of the massacre in the factory. Unbeknownst to Rachel, Alexa has also survived, and is still loose in the factory, waiting for more victims.
