Katherine, in the middle of her senior year in high school, finds herself strongly attracted to Michael, a boy she meets at a New Year's party. As their relationship unfolds, the issue of sex comes up more as an emotional and health issue than as a moral one. Both of them are aware that physical intimacy is both common and complicated.
Their relationship progresses slowly as they begin to go on dates and trips together; they are accompanied on various meetings by Katherine's friend, Erica, who has known Katherine since the 9th grade and believes that sex is a physical act and not a romantic act. Erica and Katherine are also joined by Michael's friend Artie, who, with Erica's help, explores and acknowledges some uncertainty about his own sexuality. Artie is a depressed teenager who feels life is over after high school. He shows his depression when he attempts to hang himself from his shower curtain rod but fails.
Katherine and Michael go to a Skiing Trip, where they plan to have sex, but Katherine has her period, and they are disappointed. Michael teaches Katherine how to hold his penis (nicknamed "Ralph") and how to rub it correctly. When Katherine and Michael do have sex on Michael's sister's bedroom floor, they are sure it seals a love that will be "forever". Michael buys Katherine a necklace for her birthday that says both of their names on it and it also says "Forever". However, separated for the summer by work that takes them to two different states, Katherine finds herself aware of the limitations of the relationship and is ultimately attracted to a tennis instructor, Theo, who is older and more experienced in life. She takes responsibility for breaking the news to Michael when he comes on a surprise visit and almost catches her and Theo together. Katherine realizes the "loss" of Michael, while painful at first, can be the start of new successful relationships. The book ends with Katherine's mother giving her a message that Theo called for her.