In 1955 Springfield, Massachusetts, Abby Reed Pamela Sue Martin and Muffy Pratt Betsy Slade are best friends and roommates at Penfield Academy, an all-girls boarding school. They begin their senior year trading stories about the past summer. Abby, the conventionally beautiful one of the pair, confesses she almost lost her virginity to her boyfriend Michael Parker Stevenson, who attends the nearby all-boys school St. Ambrose. An insecure Muffy regrets she has not had the opportunity.
At a mixer with St. Ambrose the following night, the pairs are chosen by luck after each girl and boy picks a number. Abby talks to Michael, who pleads with her to visit him soon. Muffy is paired with Buzzy Knight Michael Gray; he does not return her feelings and tries to switch partners with another boy. Malcolm George O'Hanlon, Jr., an awkward boy who has strong feelings for Muffy, tries to talk to her, but she is oblivious to his affections. Later, while Buzzy dances with Muffy, he holds up money to another boy in a bid to swap girls. A humiliated Muffy leaves the dance early. By night’s end, Abby promises Michael she will take an overnight trip to visit him in two weeks. When Abby returns to the dorm, she finds Muffy crying in bed. After Muffy confesses that Buzzy hurt her feelings, Abby tries to cheer her up by hinting that Malcolm is in love with her, to no avail.
Abby hatches a plan to visit Michael by posing that she is visiting her grandmother. Muffy impersonates Abby’s grandmother over the phone to headmistress Mrs. Pendleton Edith Atwater, and Abby succeeds in getting a weekend pass. Abby and Michael check into a hotel, each with a separate room. He meets Abby in her room that evening, and after some awkward moments, they consummate their relationship. Abby mourns the loss of her virginity, but Michael promises that he won’t stop loving her. Back at school, Muffy receives a letter from Buzzy apologizing for his behavior. Suspicious, Abby tries to discourage her friend from replying, but Muffy sends a response inviting him to a Christmas Eve party at Abby’s family home.
At the party, Muffy gives Buzzy her full attention and impulsively kisses him under the mistletoe. However, Buzzy reveals he is with another girl. Muffy leaves crying, with Malcolm following behind. When he finds her beside his car, she proposition him to take her virginity then; he resists, arguing she’s in the wrong state of mind, but later relents after she taunts him, and they have sex in the backseat. The encounter is not as satisfying as Abby and Michael’s. Before returning to the party, Malcolm confesses that he was a virgin until now, and Muffy says she feels no different after sex and doesn’t see “what all the fuss is about.”
Back at school, Muffy suspects she is pregnant and sees a doctor; he confirms the pregnancy. She asks if he can perform an abortion and he urges her to tell her parents, but Muffy says she cannot. He also refuses to refer her to other doctors. With few options, Muffy heads to a back-alley abortion in another city, accompanied by Abby, Michael, and Malcolm, where Michael has found someone to perform the procedure. The night before, Abby tells Muffy that Malcolm wants to marry her, but Muffy rejects the idea, insisting she wants control over her own life.
The next day, Muffy meets the doctor Frank Robert Walden, a medical student who explains the abortion will be simple and painless. After the procedure, Muffy joins Abby downstairs and seems fine. However, back at school, she becomes ill and refuses to let the nurse know what caused the hemorrhaging. Soon after, Malcolm, Michael, and Abby visit Muffy at the hospital, but discover she has died. On graduation day, Abby lies on Muffy’s bed and cries before putting on her cap and heading to the ceremony.
