Forever is told through the eyes of high school senior Katherine Danziger, who lives in Westfield, New Jersey with her artistically talented younger sister Jamie; her mother, Diana, a children’s librarian; and her father, Roger, a pharmacist and fitness enthusiast. Katherine and her smart, outgoing friend Erica Small attend Sybil Davison’s New Year’s Eve party. Sybil, Erica’s cousin, is smart, overweight, and sexually experienced. At the party, Katherine meets Michael Wagner, a high school senior from neighboring Summit who has reddish-blond hair and glasses and is an avid skier. The day after the party, Michael stops by Sybil’s house and invites Katherine on a car ride. Michael is Katherine’s first serious romantic relationship. Katherine, who is a sexually inexperienced virgin, describes her sexual awakening with Michael. Early in their relationship, they make out and explore each other’s bodies in Katherine’s den and bedroom.
Katherine and Michael also spend time with Erica and Michael’s friend Artie Lewin, a board game enthusiast who has the lead in his school play and wants to go to acting school. Erica, who is eager to lose her virginity, wants to have sex with Artie. At the cast party for the school play, Katherine worries that Michael is still interested in Elizabeth Hailey, who was his date at Sybil’s New Year’s Eve party. After the party, Michael pressures Katherine to go further sexually than she feels ready to do. Meanwhile, Erica hopes that Artie will kiss her. When Artie doesn’t kiss Erica, she asks him if he’s gay. He says he’s not sure, and she offers to help him find out.
Michael and Katherine go away for a weekend at the Vermont ski house of Michael’s sister Sharon and husband Ike, where Michael teaches Katherine to ski. They spend the night together but don’t have sex because Katherine has her period. This weekend is the first time Michael and Katherine say “I love you” to each other. While Michael and Katherine are away, Erica and Artie play strip poker, and Erica decides Artie is impotent, not gay. Later, Michael tells Katherine that Artie is depressed and hates school.
Meanwhile, Katherine’s family expresses their concerns about her relationship with Michael. Katherine’s grandmother warns Katherine about pregnancy and venereal disease and gives her information from Planned Parenthood. Katherine’s parents worry that Katherine and Michael are getting too serious. They urge Katherine to make smart decisions about sex and encourage her to date other people. Believing that Katherine needs time apart from Michael, Katherine’s mother refuses to give Katherine permission to apply to the same colleges as Michael. After a movie date, Michael tells Katherine that the previous summer, he contracted a venereal disease from the only girl he’s had sex with. When Katherine and Michael are alone at Sharon and Ike’s apartment, they have sex for the first time. Katherine insists that Michael wear a condom, though he doesn’t seem to think it’s necessary. Katherine’s first time is disappointing because Michael orgasms quickly.
Tommy Aronson, Katherine’s previous boyfriend, is home from college and calls Katherine for a date. When Katherine turns him down, he asks for Erica’s phone number. Erica goes out with Tommy but finds him boring, so she doesn’t have sex with him. On a visit to New York City, Erica and Katherine go to a Robert Redford movie. Then Katherine lunches with her grandparents, who live in the city, and later goes to Planned Parenthood to get birth control pills. On Katherine’s birthday, Michael takes her out and gives her a silver locket necklace with her name inscribed on the front and Forever, Michael inscribed on the back. The next day, Katherine and Michael have sex at Michael’s house, and this time it’s more pleasurable for Katherine.
Katherine and her friends find out about college acceptances and learn that Sybil is pregnant and will give the baby up for adoption after the birth. Unhappy with the plans that Katherine and Michael have made to stay together during college, Katherine’s father gets her a summer job as a tennis counselor at Jamie’s camp in New Hampshire. Katherine doesn’t want to go, but her parents insist. At Michael’s birthday celebration, Katherine finds out that Michael has a summer job in North Carolina. That evening, things don’t go well between Erica and Artie. Artie locks himself in the bathroom and threatens to kill himself. Erica decides she can’t help Artie and breaks up with him. Later in the week, Artie tries to commit suicide and is sent to a psychiatric hospital. In June, Sybil gives birth early and puts the baby girl up for adoption. Katherine and Michael celebrate their graduations.
Summer arrives, and Katherine and Jamie go to camp. There, Katherine exchanges letters with Michael but finds herself strongly attracted to the head tennis counselor, Theo, a kind, twenty-one-year-old college senior who seems interested in her. Katherine and Theo spend time together and become close, especially when Theo comforts Katherine after her grandfather dies. However, Theo, who wants to see Katherine after camp ends, doesn’t encourage a physical relationship with her until she is in a better emotional place.
Katherine wants to tell Michael that she feels torn between him and Theo, but she isn’t able to do so until he makes a surprise visit to see her at camp. During Michael’s visit, Katherine realizes that she doesn’t want to have sex with him, and he guesses that there’s another guy on her mind. When Katherine confirms his suspicion, Michael becomes very upset. They end their relationship, and he says hurtful things to Katherine as he drives off angrily. Back in New Jersey, Katherine runs into Michael once more before college and feels that she doesn’t regret their relationship. The story ends with Katherine returning home and learning that Theo has called.