One night during a full moon, Robert Bridgeston and his son Richie arrive at their country cabin. Inside, Robert, who is newly and reluctantly divorced from Richie’s mother Sandy, suggests a walk in the woods as an attempt to curtail Richie’s interrogation about the possibility of a reconciliation. In the forest, a werewolf attacks Richie, and Robert leaps to his aid, eventually pushing the creature over a cliff to its death, but the creature bites his arm in the process. Richie is sure the attacker was a wolf, but the dead body is that of a man. In the hospital, Robert discounts Richie’s claims to the local sheriff that a werewolf is loose. They return home the next day, where Sandy, who is still fond of Robert but has grown weary of his disdain for her career in publishing, welcomes them. Although Robert wants to stay the night, Sandy demurs, asking instead that Robert visit Dr. Marderosian, the psychiatrist Richie has been seeing to help him through the divorce. At his office, Dr. Marderosian explains that Richie likely insists upon the werewolf story because he prefers for Robert to have killed a monster rather than a man and recommends that Robert return to the cabin with Richie. Driving to the cabin several weeks later, Richie regales his father with all he has learned about werewolves, including the fact that they can only be killed by being stabbed in the heart, hit over the head with something silver, or presented with wolfsbane. They pass a camp of hippies, and soon after the sheriff also comes to investigate the camp and discovers that they are religious Christians, led by Brother Christopher, who presents the disappointed sheriff with a legal permit for assembly. That night Robert and Richie reach the fishing hole, and as Richie walks down to the water, the full moon begins to rise, and Robert transforms into a werewolf. He rushes into the woods to prowl, and when Richie glimpses the creature, he flees in terror. While Robert attacks two cars, killing the drivers by ripping off their heads, Richie runs in hysterics to a nearby trailer, where a young couple, Harry and Jenny, offer him refuge. When he insists that Harry help him search for Robert, Harry, despite disbelieving the boy’s story, wanders the woods with him, in vain. By morning, a restored Robert stumbles into their camp and reunites joyously with Richie. That night, however, Robert transforms again and goes to the trailer, where he attacks the couple and rips off Harry’s head. He returns to the cabin with Harry’s head in a sack, and buries it in the cabin cellar as Richie hides and watches. Before Richie’s eyes, Robert transforms back to himself, just as the sheriff appears to inform them of the three recent killings. In the car on the way home, Richie, confused about all he has experienced, asks Robert if he remembers what transpired the past few nights, but his father recalls nothing and denies the existence of a buried bag. At home, Richie runs into his room crying, and later tells Sandy that he is scared of Robert. When he explains about the werewolf, however, she insists he is exaggerating, and he sobs in frustration. Later, Sandy urges Robert to see Dr. Marderosian again. The doctor says that Richie is traumatized and informs Robert that if he were a werewolf, his index fingers would grow longer than his middle fingers. At the next full moon, Richie, desperate to help Robert, urges his father to return to the cabin, and Sandy agrees to accompany them. Although the previous night, Robert has transformed and murdered Marderosian, the three fail to see the newspaper headlines before venturing into the woods. When they pass the hippie camp, Robert stops, and Brother Christopher has his followers form a pentagram and urges the family to step inside and renounce Satan. When Robert tries to enter it, however, he cannot walk forward, prompting Christopher to call him a devil. That night, as Sandy informs Robert that she wants to get back together with him, he rushes out, feeling himself transforming. Meanwhile, Richie sneaks into the basement to dig up the buried bag, and finding him there, his father begs him to lock him in before he hurts anyone. Sandy spots Richie locking the basement door, however, and tries to unbolt it, until she sees Robert’s claw burst through the wall. She grabs Richie and drives away, followed by Robert. They flee to the hippie camp, where everyone is asleep on the ground. Upon hearing a wolf howl, Brother Christopher believes he is being tested and awakens his followers, who are soon assaulted by the werewolf. Robert chases Brother Christopher until he sees the sun rising, then collapses and transforms. At the cabin, the sheriff is investigating the claw marks, frustrated by Richie’s insistence that the attacker is his father, in werewolf form. Robert returns, noting in desperation that his index finger has grown long and clawed. Both the hippies and the townsmen descend on the sheriff’s office, demanding action, but the sheriff turns them away. In the cabin that night, Robert transforms once again and fondles Sandy, who awakens and screams. As Robert escapes out the window, the police guard stationed outside shoot at him. The sheriff agrees to form a posse, which surrounds the area, and Richie convinces Sandy to go along in order to protect Robert, whom she is beginning to suspect may be involved. Hearing a howl from the woods, Richie runs toward the sound, and upon spotting his father, warns him to flee. Robert takes the boy’s hand and races away, followed by the sheriff, Sandy and a large group of armed men. The men find Robert and surround him, joined by the hippies. As Robert scoops up his son, he is shot, and as Sandy cries out, Robert tosses Richie to safety. The posse shoots dozens of bullets into Robert, who does not die until he finally falls onto a stake that punctures his heart. When the group gathers around to examine him, his face reverts to Robert’s. As Richie sobs, Sandy screams in horror.