Bill Nugent Michael Cutt, an anthropology professor, is approached by Carla Thomas Shannon Cooper, a young woman whose father was murdered in the woods of Northern California. She believes the killer was a Bigfoot said to reside in the region. Despite a chilling string of disappearances and brutal murders—including the death of a young couple, the manual emasculation of a biker, and the butchering of two Girl Scouts—police dismiss the Bigfoot theory as a hoax. Carla asks to accompany Bill on an expedition with four of his students—Roy Bob Collins, Pete, Gary, and Linda Jody Lazarus—to prove that the creature exists.
They travel by boat along a river and first camp on the property of an elderly man, Lou Carlson William F. Nugent, who claims to have witnessed Bigfoot but is evasive when pressed. When one of the students, Roy, bribes Lou with alcohol, Lou suggests they seek out Wanda, a recluse who lives deep in the woods and supposedly holds more information. A hunter camping nearby is attacked and impaled by the Bigfoot on a jagged branch.
The next day, Nugent and his five companions visit the nearby town and learn more about Wanda: she’s said to be the daughter of a deranged preacher, Emmet McGinty, whose followers were rumored to engage in inbreeding, cannibalism, and human sacrifice. Emmet has since committed suicide. After learning of this, the group returns to the woods. That night, they are awakened by chanting. Bill and Roy follow the sound and stumble upon a Satanic sexual ritual involving Wanda around an effigy of Bigfoot; among the practitioners is a local sheriff Paul Kelleher. Bill interrupts the ritual by firing a gun, scattering the cultists.
In the morning, their boat is missing, and large footprints mark the shore. They make plaster casts of the tracks and press on. That night, the Bigfoot attacks Linda and Gary while the two are together, but they survive. The group eventually reaches Wanda’s cabin, bribing her with candy, but she has a nervous breakdown when the Bigfoot is mentioned. That same night, Pete is attacked by the Bigfoot, who causes him to shoot himself with his rifle. The others search for Pete, but he has disappeared.
Bill hypnotizes Wanda the next day, prompting her to recall a brutal childhood memory: as a teenager, the Bigfoot raped Wanda—a moment Emmet had witnessed. Believing the beast to be a demon, Emmet tried to induce an abortion by forcing Wanda to drink poison. Wanda ultimately gave birth to a mutant half-human child, but Emmet killed it. In retaliation, Wanda burned her father alive in his barn, staging it to look like a suicide.
Hoping to find evidence of Wanda’s Bigfoot child, Bill digs up the skeletal remains of the infant in a cemetery nearby. The Bigfoot then appears in the graveyard and absconds with its dead spawn as Bill and the others barricade themselves in Wanda’s cabin. Hours later, the Bigfoot hurls Pete’s mutilated body onto the porch, then breaks into the home. Wanda remains disarmingly calm as the creature strangles Carla to death while the others hide in the kitchen. The Bigfoot breaks into the kitchen, where it disembowels Gary; Roy tries to stop it, but the beast smashes his head through a window and slits his throat on the glass before impaling Linda with a pitchfork. Finally, the Bigfoot burns Bill’s face on a hot stove, leaving him for dead.
Sometime later, Bill lies in a hospital bed, unconscious in the woods, after being found near the scene with his face burned. When detectives question the missing Carla and the four students, Bill recounts his version of events, but both the police and the attending doctors doubt him, and a hospital psychologist deems him criminally insane.
