Two technicians in a sophisticated industrial facility—Richard Sitterson (Richard Jenkins) and Steve Hadley (Bradley Whitford)—are getting ready for an unknown operation, one of several taking place around the world.
Meanwhile, five college students—Dana (Kristen Connolly), Curt (Chris Hemsworth), Jules (Anna Hutchison), Marty (Fran Kranz), and Holden (Jesse Williams)—drive out for a vacation to a remote cabin in the woods, while the technicians keep an eye on the students with hidden cameras. Through the use of sophisticated environmental controls and the release of mood-altering drugs into the air, each student is manipulated into a fairly common horror archetype by the organization, a point which is made several times in the film itself. These changes generally downplay the students’ intelligence, while enhancing sexual libido and lack of caution.
At the cabin, the friends settle in while the observing technicians place bets on which scenario the students will unwittingly select. Once the students enter the basement, they discover a number of artifacts, each of which hints at its own sub-genre of horror, ranging from a sphere which is reminiscent ofLemarchand’s box from the Hellraiser series to a scarab-like pendant. Dana reads an incantation from an old diary, which is the trigger for a scenario involving a family of zombies which rise from the grave outside, though the students remain unaware of the danger they have released.
Curt and Jules go outside to have sex. Zombies attack the lovers and Jules is killed. Meanwhile, in the cabin, Marty becomes paranoid and his fears are confirmed when he finds a surveillance camera in his room. Before he can act, however, he is attacked and presumably killed off-screen. Holden, Dana, and Curt flee in their RV and get as far as a long mountain tunnel, but their escape is thwarted when the technicians blow it up. While trying alternate routes of escape, Curt and Holden are killed.
The technicians drink in celebration that they have completed the “ritual,” while the screens in the background show Dana fighting for her life against the zombies. The party stops when a phone call from “upstairs” informs them that Marty is still alive, just as he appears and rescues Dana. Marty, having survived his encounter with the zombies, came across a hidden control box near the cabin from which he was able to access an elevator with glass walls. He and Dana take it down to the facility where the technicians are, and as they do so, they pass by a large number of horror genre monsters, each in their own glass cage. Dana and Marty escape the elevator, but are cornered by a security team. To escape, they release the monsters in all the pods, who start killing everyone in the facility.