Jim and Lori, a young couple, crash their motorcycle in the Mill Creek region of the California redwood forest, leaving Jim with an injured leg. Lori searches in vain for help, and when she returns to the scene, Jim is hanging upside down from a tree with his throat cut. She flees in terror, tripping a wire that deploys a pair of tree branches covered in sharpened tin-can lids. Days later, at the barracks of the Redwood Youth Corps, Marco Cerone, Dennis Zorich, Nathaniel Hines, and Boone are rudely wakened by Eggar, the mechanic, who takes pleasure in taunting the young volunteers. In the kitchen, Sammie, the cook, complains to forest ranger Mike that several large cans of peaches are missing from the pantry. Mike dismisses the complaint as he is preoccupied with his team’s impending work detail in Mill Creek, which will include his girl friend, Melanie, and three of her female friends. The volunteers also look forward to the trip, but are opposed to having the bullying, erratic Eggar drive the bus. Mike promises that Eggar will behave, but when he tries to speak to the mechanic, Eggar argues that the trip is dangerous and unnecessary, citing the recent disappearance of Jim and Lori in the same vicinity. Regardless, Mike asserts his authority and the expedition proceeds. Along the way, they pick up Melanie, Margaret, Vanessa, and Windy Morgan, and pass a mental institution where Eggar was rumored to have been an inmate. When the bus reaches its destination, Mike rejects Eggar’s argument that the terrain is too rugged for the women, and the group gets to work removing debris from the creek. During a break, Marco questions rumors of marijuana fields in the vicinity. Boone confirms the rumors, but warns that the farmers are likely to shoot trespassers on sight. When the men tease Eggar about his inability to “get high” due to a lack of brain cells, the driver tells the story of how he once bound a man and left him for dead in the nearby hills. That evening, as the group sits around the campfire, Boone relates the legend of a young girl who went insane after being raped by her uncle. She was placed in a mental institution, where it was discovered that she was pregnant. The child was raised by another family, but returned nineteen years later to arrange for his mother’s release. Realizing that her psychosis would prevent her from functioning in society, the son set her loose in the redwood forest, where she continues to torment campers. Eggar is angered by the story, and as he returns to the bus, Mike instructs him to meet the group downstream the following day. Dennis, Boone, and Nathaniel plan to sneak away from camp to steal some marijuana plants, and only allow Marco to join them on the condition that he acts as lookout. Marco agrees, unaware of his companions’ plan to abandon him in the woods. The next morning, Mike notices Marco’s absence and orders the other men to find him. Mike and Melanie join the search, and once they are alone by a pond, they make love in the water. Suddenly, an unseen assailant kills Mike with a machete and takes Melanie prisoner. Meanwhile, Nathaniel and Dennis come upon a cabin where they discover a government-issue rubber raft, large cans of peaches, a jar of psilocybin mushrooms, and a hat belonging to Eggar. Believing their driver may be responsible for Marco’s disappearance, they take refuge at the campsite. Marco reaches the site by nightfall to find the others distraught over the missing members of their party. When Vanessa finds Mike’s severed head in the outhouse, Dennis, a Vietnam War veteran, rallies the others for battle against Eggar. In the morning, the campers return to the cabin and steal Eggar’s raft, unaware that Melanie is being held captive in a crawlspace underneath the floor. When they leave, a grotesque figure emerges from the crawlspace dragging Melanie behind. As the group rafts downstream toward the coast, Melanie’s body is dropped from a tree. Her horrified companions bury the body and continue on their trek. Hours later, they come upon the bus and climb aboard in hopes of hastening their escape, then realize that the vehicle is disabled. The group decides to spend the night on the bus, until the killer appears and forces the campers back into the woods. Windy is separated from the others and the killer slashes her face. After Margaret stitches the wound, the group devises a plan of attack. The next morning, they execute their plan without Dennis, who is exploring a crater while intoxicated on mushrooms. Marco stands in a clearing and calls to Eggar, with the intent of provoking a confrontation. Eggar grabs Marco from behind before being set upon by the other campers. Dennis moves to higher ground, but the killer pulls him back into the crater, and he dies in the fall. Eggar reveals the killer to be the rape victim of the legend, and also his mother. She runs toward the scuffle and trips a rope, resulting in her impalement on a trapeze of sharpened sticks.