In an underground laboratory, engineers Sitterson and Hadley discuss plans for a mysterious ritual, after a similar operation in Stockholm has just ended in failure. American college students Dana, Holden, Marty, Jules, and Curt are spending their weekend at a seemingly deserted cabin in the forest. From the lab, Sitterson and Hadley remotely control the cabin and manipulate the students by intoxicating them with mind-altering drugs that have effects such as hindering rational thinking and increasing libido. The lab departments take bets on what kind of monster will attack the students and discuss the failures of international operations. In the cabin’s cellar, the group finds many bizarre objects, including the diary of Patience Buckner, a cabin resident abused by her sadistic family. Dana recites incantations from the diary and inadvertently summons the zombified Buckner family.
Hadley releases pheromones to induce Curt and Jules to have sex outside. They are attacked by the zombies and Jules is decapitated while Curt escapes to alert the group. Marty discovers concealed surveillance equipment in his room before being dragged off by a zombie. The lab workers learn that the rite in Japan has also failed, meaning that the American rite is “humanity’s last hope”. Curt, Holden, and Dana attempt to escape in their RV, but Sitterson triggers a tunnel collapse to block them. Curt attempts to jump a ravine on his motorcycle to seek help on the other side, but crashes into a force field and falls to his death. Holden and Dana realize that their experience is staged and retreat to the RV. As they drive back to the cabin, Holden is stabbed by a zombie, and the RV is driven into a lake. Dana swims ashore and is also attacked. The lab employees, seeing that Dana is the only survivor, celebrate the success of the rite, but are interrupted by a phone call from “The Director” pointing out that Marty is still alive.
Marty rescues Dana and takes her to a hidden elevator he discovered under a grave. They descend into the lab and discover a large collection of monsters of various types locked in cages. Dana correlates them with the objects in the cabin’s cellar and realizes that the objects determine which monsters are released. Cornered by security personnel, Dana and Marty trigger a “Purge” button and release all the monsters, which wreak havoc and slaughter most of the staff. Dana and Marty flee the carnage and discover an ancient temple, where they are confronted by The Director. She explains that worldwide annual rituals of human sacrifice are held to appease the Ancient Ones, a group of cruel subterranean deities. Each region has its own ritual, and the American ritual involves the sacrifice of five slasher film archetypes: the whore (Jules), the athlete (Curt), the scholar (Holden), the fool (Marty), and the virgin (Dana). The order of the killings is arbitrary as long as the whore dies first and the virgin dies last or survives. The Director urges Dana to kill Marty to complete the ritual and spare humanity. Dana is suddenly attacked by a werewolf, while Patience Buckner, one of the zombies, kills The Director. Deciding that humanity is not worth saving, Dana and Marty share a joint while awaiting their fate. The temple floor collapses and a giant hand emerges, destroying the facility and the cabin itself.