An experimental drug that is supposed to reduce aggression has the opposite effect on a baboon that is being experimented on by a group of medical students and their professor. While they are playing a fantasy role-playing game in their research facility, the baboon escapes and begins hunting them down.
As a child, Luther Watts was deeply affected by seeing a circus geek (someone who bites the heads off live chickens in a circus sideshow). Finally paroled after serving 20 years in prison, he terrorizes the residents of his hometown by making chicken noises, crowing like a rooster and attacking people. He winds up at a farm run by a woman and her daughter, where he takes them captive and then starts killing off her neighbors. The mother realizes she and her daughter must escape before he kills them, too.