Dawn O’Keefe (Jess Weixler) is a teenage spokesperson for a Christian abstinence group called The Promise. She attends groups with her two friends, Gwen (Julia Garro) and Phil (Adam Wagner). One evening after giving her speech talking about the purity ring those in the group wear and what it means, she is introduced to Tobey (Hale Appleman) and finds him attractive.
The four begin going out as a group, and Dawn has fantasies of marrying Tobey, although after acknowledging the attraction they agree that they cannot spend time together. Soon after they give in and meet at a local swimming hole. After swimming together, they go in to a cave to get warm and begin kissing. Tobey then forces himself on Dawn; in the panic her vagina bites off his penis. Horrified, he stumbles away and she flees the scene. After a Promise meeting, she meets her classmate Ryan (Ashley Springer) at a dance; they talk, and he drops her off at her home.
Dawn researches "vagina dentata" and realizes she may have it, then she visits a gynaecologist, Dr. Godfrey (Josh Pais) in an attempt to find out what is happening to her. Finding out that she has never visited a gynaecologist before, he attempts to take advantage of her by saying he is performing a test, when what he is really doing is molesting her. She panics and her vagina bites off the fingers on his right hand. On her way back, she sees someone driving in Tobey's car, and she goes back to visit the pool. When she gets there she sees the police bringing up Tobey's body. At home, her ill mother, Kim O'Keefe (Vivienne Benesch) collapses, Dawn's stepbrother, Brad (John Hensley) and his girlfriend Melanie (Nicole Swahn) ignore her and continue to have sex while she lies on the floor. Kim is taken to the hospital.
Dawn goes to Ryan seeking help and the two successfully have sex. The following morning they have sex again, but mid-coitus Ryan’s friend calls, and Dawn learns that he had a wager on having sex with her. In her anger, her vagina bites off his penis, and she leaves him to call his mother for help.
Dawn learns her mother has died, and after her stepfather Bill (Lenny Von Dohlen) attempts to throw Brad out, Brad sets his dog on Bill. Dawn meets her stepfather and Melanie at the hospital, and emboldened by her power, she goes back home to seek revenge. Dawn puts on make-up and goes to her stepbrother, and they engage in sex. In the middle of the act, Brad recalls when he was younger and his finger was bitten by Dawn, that it wasn’t her mouth that bit him. As he realizes this, Dawn’s vagina bites off his penis. She releases it on the ground, and Brad’s dog eats it, spitting out the glans and genital piercing. Dawn leaves him to bleed to death.
Dawn cycles away from home, but her bike tire sustains a puncture, so she begins hitchhiking. She gets a lift from an old man (Doyle Carter), but when she reaches the next gas station and tries to get out, he locks the doors. He licks his lips as if to ask for a sexual favor to release her; Dawn hesitates, and then looks towards the camera -and the old man- with a seductive smile.