https://www.youtube.com/watch?v=D2tnp4QCKtk

Shakma, a hamadryas baboon belonging to medical student Sam, is injected by Sam's professor Sorensen with an experimental variation of corticotropin intended to inhibit aggression. When Shakma awakes and goes berserk, Sorensen instructs Sam to euthanize him, but Sam cannot bring himself to carry out the order and leaves Shakma unconscious. Before fellow student Richard can cremate Shakma, Sorensen insists on performing a necropsy the following day, and Shakma is left alive. That night, Sorensen organizes a live-action role-playing game for Sam and his colleagues Gary, Bradley, Richard and Tracy. The objective of the game is to reach Richard's younger sister Kim on the top floor. The players are issued walkie-talkies, which allow communication with only Sorensen, the gamemaster. As part of the game's preparation, Sorensen disables the fire alarm system and locks the offices and building to disallow any player from leaving until the end of the game. Shakma, who has since regained consciousness, kills Bradley when he ventures into the specimen room on the fifth floor. Richard is also killed while investigating Bradley's whereabouts on Sorensen's instruction.

Sorensen investigates the players' disappearances personally and finds Richard's mutilated body before falling prey to Shakma himself. The sounds of Shakma's carnage alerts Sam and Tracy to his presence, but they are unable to use the elevator as Sorensen's body is jamming the door. Sam takes the stairs to the fifth floor and unsuccessfully attempts to tame Shakma before fleeing the floor along with Tracy. Tracy goes to the electricity lab to collect a strobe light that she uses to temporarily blind Shakma, and she holds the specimen room door shut while Sam goes to locate and remove Sorensen's body from the elevator. When Tracy confirms Richard's presence in the specimen room, she distracts Shakma while Sam goes to recover Richard, but Shakma returns before he is able to recover from the shock of discovering his body, and Sam and Tracy narrowly escape Shakma's pursuit once more. As Sam and Tracy begin a search for Gary and Bradley, Gary is killed while using the elevator, which allows Shakma onto the sixth floor. Tracy is killed in the floor's restroom shortly after.

Sam goes to the top floor to warn Kim (without letting her know of Richard's death), arms himself with a kitchen knife and continues his search for the others while Kim stays to attract the attention of Richard's girlfriend Laura, whose car is parked below. As Sam finds Gary's and Tracy's bodies as well as Sorensen's keys, Laura drives away, and Kim decides to go find Richard herself after leaving a hand-written note for Sam; she is killed by Shakma after finding Richard's body. Sam, after finding Bradley's and Kim's bodies, prepares to alert the police from Sorensen's office, but his grief over Tracy's death, in particular, drives him to kill Shakma himself. He sets up a trap to electrocute Shakma, who avoids the trap and mortally injures Sam. Shakma is ultimately burned to death when Sam manages to lure him into the incinerator. Sam collapses in a hallway and declares "I win" to a nearby monkey doll, before dying.