Dawn O’Keefe (Jess Weixler) is a teenage spokesperson for a Christian abstinence group called The Promise. 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 Toby (Hale Appleman) and finds him attractive. The four begin going out as a group, and Dawn has fantasies of marrying Toby, 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. Dawn gets uncomfortable and tries to get them to go back. Toby then attempts to force sex on Dawn, who panics and tries to push him off. Toby becomes aggressive and shakes Dawn, resulting in her smacking her head on the ground. While she is recovering from disorientation, Toby 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 researches "vagina dentata" and realizes she may have it. She then 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 Toby's penis.