Dawn O'Keefe, Jess Weixler, is a teenage spokesperson for a Christian abstinence group, “The Promise.” She often hears her stepbrother Brad arguing with his girlfriend Melanie about sex, with Brad insisting on only anal intercourse and spurning vaginal sex.
One afternoon, Dawn joins a group meeting where she meets Tobey. Dawn, along with her friends Gwen and Phil, begins dating Tobey as part of the group dynamic. Although there is obvious attraction between Dawn and Tobey, they initially agree not to spend time alone together. That rule fractures when they do sneak off to a local swimming hole. After swimming, they seek warmth in a cave and start kissing. Dawn becomes uneasy and tries to coax Tobey to leave, but he becomes aggressive. When Dawn tries to push him away, he shoves her and slams her head to the ground. As Tobey tries to rape her, Dawn fights back, and in a shocking moment, his penis is severed as it enters her vagina. Horrified, Dawn flees the scene.
Back at a Promise meeting, Dawn encounters classmate Ryan at a dance. He offers to take her home and asks her out, but she declines. Dawn then returns to the swimming hole and is horrified to find Tobey’s severed penis gnawed on by a crab. In a symbolic act, she drops her purity ring from a cliff. She begins researching the myth of vagina dentata and wonders if she possesses it. A visit to gynecologist Dr. Godfrey leads to a traumatic examination in which he assaults her by reaching up inside her without protection. In a panic, Dawn’s body responds in self-defense, and she bites off four of his fingers. On her way home, she notices police vehicles and a car that resembles Tobey’s; that evening, Tobey’s body is recovered.
Dawn, overwhelmed and unstable, seeks help from Ryan, who provides a sedative and uses a vibrator to help her relax. When she is calm and consenting, Dawn discovers that her teeth do not engage. The next morning, they have sex again, but Ryan boasts to a friend that he had bet he could coax her into sex. Enraged, Dawn’s vagina bites off his penis, and she leaves as he screams.
Dawn’s stepfather Bill tries to eject Brad from the house, but Brad retaliates by siccing his dog on Bill, declaring his love for Dawn. Dawn goes to the hospital after her mother Kim falls ill, and she learns from Melanie how Brad told her to ignore their mother’s pleas for help. Fueled by a sense of power and vengeance, Dawn returns home and plans revenge. She dons makeup and goes to Brad’s room to seduce him. A childhood memory surfaces in which Dawn bit Brad’s finger; Brad realizes that it was not Dawn’s mouth that bit him this time, and his own memory confirms the terrifying truth: Dawn’s vagina bites off Brad’s penis. The dog does not attack—rather, it consumes the severed organ, spitting out the glans. Dawn leaves Brad’s pleas for mercy behind.
She hops onto her bicycle, only to suffer a punctured tire and hitchhike away. An old man offers a ride, but Dawn falls asleep and awakens after nightfall outside a convenience store. When she tries to exit, the man locks the doors and licks his lips. Dawn hesitates, then turns to the old man with a sly, unsettling smile, hinting at a dangerous future.
