After shooting up heroin, a woman commits suicide. Clips show the history of the Exeter School of the Feeble Minded. These document its abuses toward its patients, demise, and eventual abandonment. Years later, Patrick volunteers to help Father Conway renovate the site. Conway, who is disappointed that Patrick has not applied for college, believes that God has a special plan for him. After talking to local junk man Greer, Conway and Patrick leave for the weekend.
When Patrick's friends discover the site will be unsupervised for the weekend, they organize a huge party there over Patrick's objections. As the party winds down, seven people remain: Patrick, his younger brother Rory, Brian, Brad, Amber, Drew, and Reign, a girl that Patrick has met at the party. After they discuss the site's troubled history and its potential for supernatural phenomena, Amber convinces the others to attempt to levitate Rory. Amber says it worked, but the others dismiss it as a prank by Rory, who they say is attention-starved. Embarrassed that he peed his pants, Rory runs off.
When Patrick and Reign encounter Rory later, they become convinced that he is possessed, as he is speaking in tongues and viciously attacking anyone who approaches him. After restraining him, Reign convinces Patrick to call Father Conway for help. The others react angrily, as they have been ingesting a variety of illegal drugs. As they argue, Greer returns and threatens to call the cops unless Amber has sex with him. Greer hears a sound from upstairs and investigates, only to be killed by Rory. Brian and Amber panic, and as they flee Exeter, they accidentally strike Father Conway with their car, apparently killing him. The car does not make it off the grounds.
As the teens debate what to do next, Exeter's security system locks them in. Having two dead bodies and a pile of drugs, the teens agree to resolve the situation themselves. Using their cell phones, they find a do-it-yourself exorcism guide and perform it on Rory. Although it initially seems to have succeeded, Rory reverts to his possessed self. Looking for clues to help them, Reign and Patrick discover video tapes of a troubled teen at Exeter, Devon, who is apparently Conway's child. A message written in blood on one of the ceilings demands that the teens speak to the entity, so they carve out a Ouija board. The entity identifies itself as Devon and demands they save it from a box, where it is trapped.
Rory recovers from his possession and does not remember anything. At the same time, Amber shows signs of possession. She kills Drew before the others drive her off. Brad, frustrated by the attacks, arms himself with a pickax and looks for Amber. He is surprised when he sees Father Conway, trips, and accidentally impales himself. Reign and Patrick go after Amber while Rory and Brian guard the downstairs area. Rory leaves to urinate, and Amber attacks Brian. After killing Amber, Patrick and Reign find Rory, who says that Conway trapped him in a box for his own protection. The group encounter Brian, who is now possessed; Patrick kills Brian after he attacks them.