Dawn O'Keefe (Jess Weixler) is a teenage spokesperson for a Christian abstinence group called The Promise in some small Midwest town. She attends groups with her two friends, Alisha (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.
One day, Dawn and Tobey and their friends meet at a local swimming lake. After swimming together, Dawn and Tobey go in to a cave to get warm and begin kissing. Dawn gets uncomfortable and tries to get them to go back. Tobey then attempts to force sex on Dawn, who panics and tries to push him off. Tobey becomes aggressive and shakes Dawn, resulting in her smacking her head on the ground. While she is recovering from disorientation, Tobey takes this as his chance to begin raping her. Dawn fights back and inadvertently bites off his penis with her vagina. 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 drops her purity ring off of a cliff and even goes back to the swimming hole, only causing her to scream in horror because a freshwater crab was crawling on Tobey's penis. She then researches the so-called myth of "vagina dentata" (teeth in her vagina) and realizes she may have it. Then, she visits a gynecologist, Dr. Godfrey (Josh Pais) in an attempt to find out what is happening to her. Finding out that she has never visited a gynecologist before, he takes advantage of her and molests her under the pretense of performing a test. Crying out in pain, she panics and her vagina bites off four of the fingers on his right hand.
On her way back, she sees a police officer driving Tobey's car, who demands that she pull over to the side of the road on her bike. After, she decides to go back to visit the lake to investigate. When she gets there she sees the police bringing up Tobey's dead body. At home, Dawn's ill mother, Kim O'Keefe (Vivienne Benesch) collapses. Dawn's brutal and misogynist 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 in mid-coitus Ryan's friend calls, and Dawn learns that he had a wager on having sex with her. In her anger, her body bites off his penis, and she leaves him to call his mother for help.
Dawn goes to the hospital where she learns her mother has died, and after her stepfather, Bill (Lenny von Dohlen), attempts to throw Brad out, Brad sets his dog on Bill and mangles him. Dawn meets her stepfather and Melanie at the hospital, and after hearing Melanie explain that she heard her mother screaming but Brad told her it was not a big deal, she goes back home to seek revenge. At the hospital, Dr. Godfrey is seen going into surgery to have his four fingers re-attached to his hand, and afterwords, Ryan is also there to have his penis re-attached. Both Dr. Godfrey and Ryan refuse to give an explanation to the attending doctors about their conditions.
Meanwhile, 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 was molesting her. As he realizes this, Dawns 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 gets on her bicycle and cycles away from home and out of town to search for a new life for herself, but her bike tire sustains a puncture, so she begins hitchhiking to go on the run from the law. She gets a lift from a leering old man (Doyle Carter), but that evening when she reaches the next gas station and tries to get out, he locks the doors. The dirty old man 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 sinister smile...