Horace Bones, Bhaskar Roy Chowdhury, the leader of a Manson-like cult of hippies, conducts a Satanic ritual in the woods. Local girl Sylvia, Arlene Farber, who has befriended cult member Andy, secretly observes the preparations. Sylvia is noticed by the cult and flees, but she is quickly captured and assaulted by several of the cultists. She staggers out of the trees the next morning, battered and deeply shaken. Her younger brother Pete, Riley Mills, and Mildred, who runs the local bakery, find Sylvia and take her back to their grandfather, Doc Banner. Mildred seeks help from her boyfriend, Roger Davis, John Damon, the leader of a construction crew working on a nearby dam. With the town’s closure looming and most of it deserted, the cult’s van breaks down, forcing the group to stay in town and make use of vacant buildings wherever they can.
When Banner learns of the attack on Sylvia, he confronts the cult, only to be assaulted and drugged with LSD. Horace initially intends to kill Banner, but Pete and cult member Sue-Lin, Jadin Wong, intervene, and Banner is released. Enraged by the assault, Pete grabs a shotgun and goes out looking for vengeance, but he encounters and kills a rabid dog instead. In a grim turn, Pete collects some of the dog's blood and injects it into meat pies at Mildred’s bakery, then sells the pies to the cultists. After they eat the contaminated pies, the infected begin to show signs of illness and slip into escalating violence.
One cult member panics and flees into the night. She is picked up by construction workers sent by Roger to look into the situation. She takes part in a party, has sex with some group members, and then begins to show signs of infection. Molly, Rhonda Fultz, and Carrie, Lynn Lowry, also escape with the group. Two construction workers investigate the house the cultists occupy, and Horace kills them.
Andy and Sylvia, who have managed to make their peace, are found at the Banner house by Pete, who admits what he has done. Banner reports a potential rabies outbreak and is joined by Dr. Oakes, David E. Durston, the town doctor. Banner, Oakes, and Roger soon realize that the entire construction crew is infected, and the mob closes in as the infected stumble toward a water-filled quarry, where fear overruns aggression and the attackers retreat.
Molly and Carrie reappear, taken in by a concerned homeowner, but Carrie quickly exhibits signs of rabies and attacks the resident. Andy helps Sylvia and Pete escape after Banner is found dead in the barn, impaled on a pitchfork. They encounter Molly, who chooses to end her life after learning she is rabid. Horace encounters Sue-Lin, who escapes by suicide via immolation, and Rollo, George Patterson, battles Horace, allowing Andy, Sylvia, and Pete to flee. Rollo gains the upper hand and impales Horace with a sword.
The trio then encounters Mildred, who has barricaded herself in the bakery. When Mildred opens the barricade, Andy is killed by a rabid construction worker. Sylvia and Pete retreat with Mildred to the basement, where they fend off a rabid townsfolk who enters and is killed by Mildred. The group leaves the bakery in Mildred’s car, but they are soon overwhelmed by crowds of rabid cultists and residents. Oakes arrives with reinforcements and guns down the infected. Mildred, Sylvia, and Pete emerge from the car shaken but otherwise unharmed, reflecting the quiet after a night of terror and the fragile line between civilization and chaos.
