After being cursed by their mother Petranella, Tra and Gar, a caveman brother and sister, are forced to live their lives preying on young people by devouring their entrails to retain youth. The nature of the curse allows it to be lifted after 12,000 years—a full cycle of the zodiac—after which one of the siblings must conceive a child to use as a virgin sacrifice in order to achieve immortality.
Millennia later, in 1965, the siblings subsist by stalking young people in a Los Angeles park, consuming their entrails to imbibe their lifeforce. As the 12,000 years reach their end, Gar plans to conceive a child. Using the name Mark, he rents a room in the home of a beautiful young woman, soon marrying her before conceiving a daughter, Bondi. Mark becomes obsessed with Bondi's welfare, and his attention to her causes his marriage to disintegrate in the intervening years. On Bondi's sixteenth birthday, Mark gives her an ancient amulet that belonged to his mother. During her birthday party, Bondi's mother storms out of the house, abandoning them.
Bondi subsequently runs away, hitching a ride with three adult male hippies. When they attempt to gang rape her in the back of the van, Bondi invokes her father, clutching to the amulet, which begins glowing. The van begins to drive out of control, crashing off a rural bridge before bursting into flames; Bondi survives, however, having managed to escape before the crash. Bondi wanders into an abandoned house near the park, where Tra, withering from her lack of sustenance, resides in seclusion. Bondi is unaware that Tra, who calls herself Patty, is in fact her aunt. Tra immediately realizes who Bondi is upon seeing the amulet.
Bondi is met by Nick, an 8-year-old boy who has been taken in by Patty, and who sees her as a grandmother figure. She also meets Cowboy, another runaway who lives there. Bondi is initially terrified by Patty's grotesque appearance, but Nick and Cowboy assure her she is harmless. Cowboy explains that Patty dons a cloak so as to appear as a witch, and has concocted a tall tale about the land suffering "Petranella's Curse" in order to keep people away. Meanwhile, while selling flowers on the street, Nick meets Taft, a local writer and historian who befriends him. In conversations, Taft explains the dark history of the local park, and the deaths that have occurred there for centuries, warning Nick not to go near it.
Late at night, Nick witnesses Patty entering the park in a mask, and follows her. He watches from a distance as she strangles, disembowels, consumes a female victim. When she removes the mask, her youthful appearance is restored. Horrified, Nick flees back to the house to notify Bondi and Cowboy, but finds the house empty. Meanwhile, Bondi, having been drugged by Patty, awakens in a cave alongside Cowboy. She is confronted by her father, who attempts to force her to strip nude to begin the ritual, but Patty intervenes and stops him. After a fire breaks out in the cave, Patty urges Bondi to swallow the amulet, which she does. Upon doing so, Bondi becomes possessed by Petranella, taking on her withered appearance. Petranella reanimates the corpses of Patty and Mark's victims, which they have stored in the cave, and forces them to kill and consume Mark and Patty.
After Petranella's spirit leaves Bondi's body, Bondi and Cowboy flee, and find Nick in one of the tunnels. They escape through a crumbling cave wall with the help of Taft, who has been searching for Nick in the park. The three spend the night at Taft's home, and the next day return to visit Patty's home. Upon arriving, they are notified by a city official that the home is being demolished. Pondering where to go, the three venture into the park and begin playing at a playground. Nick climbs a slide, and asks Bondi to push him down. Instead, Bondi begins to dig her fingernails into his abdomen to disembowel him. While doing so, she knowingly smiles.
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