Narrated retrospectively by its adult protagonist, the novel follows fifteen-year-old Mina through a single Ohio summer that transforms her understanding of love, friendship, and desire. Mina lives in Doan, an affluent residential neighborhood, with her parents. Her mother, Celeste, is serious and morally exacting. Mina's world revolves around her two best friends: Margaret, whom she has known since childhood, and Eleanor, who joined their friendship in seventh grade. The three share an intense intimacy, choosing each other's outfits, sleeping in each other's beds, and disclosing every detail of their emerging sexual awareness. Margaret, whose parents are mid-divorce, enjoys the most freedom among them. Eleanor's mother, Kim, an interior designer, imposes few rules. Mina's mother, by contrast, is vigilant and protective.
The summer begins with Margaret announcing she has given oral sex for the first time, to a disposable older boy from another school. She recounts the experience in graphic detail, even demonstrating the positioning with Mina as a model while Eleanor watches with flushed attention. Mina observes that Margaret embellishes and adjusts the story between tellings. Privately, Mina recalls Ginevra, a fairy from an erotic blog she reads obsessively. When the three play
The Sims, a life-simulation video game, their gameplay carries an unspoken charge: Eleanor directs Mina to make her Sim pursue romance, and after Eleanor leaves, Mina masturbates.
Subtle tensions emerge. Mina discovers through social media that Margaret went out with her older cousin Bea without mentioning it. When confronted, Margaret tells the story easily, without apology. The three dress up elaborately on a Friday night, but no plans materialize. Margaret learns her mother, Nancy, is spending the night on a date, which unsettles Mina. Eleanor proposes they sleep outside at the high school. They make a bed of blankets on a grassy hill beyond the soccer field, whispering secrets in the warm dark. Mina wakes very early to find Eleanor standing nearby. Eleanor kisses her, one hand on her waist, the other touching her neck. The kiss is brief. Eleanor's face shows extreme vulnerability. She tells Mina to say goodbye to Margaret for her and walks away through the field.
Mina arrives home elated and immediately tells her mother about the kiss. Celeste's first response is alarm that Mina slept somewhere other than where she said she would be. A bitter fight erupts. Her mother acknowledges the kiss is wonderful but announces she will call the other parents to report the overnight excursion. Mina texts her friends to warn them. Margaret responds briefly; Eleanor sends nothing.
The consequences vary sharply. Nancy yells at Margaret but mostly out of embarrassment; Margaret cleans the basement. Kim tells Eleanor to mention it next time she plans to go "camping." Mina's mother forbids sleepovers and socializing for three weeks. In the silence, Mina texts Eleanor obsessively, but Eleanor never responds. By Sunday, Mina deletes every unanswered message. She concludes that whatever Eleanor wanted, she no longer wants. During her confinement, Mina discovers subtitled versions of
Sailor Moon, an anime she and Eleanor both love, and learns that two characters presented as cousins in the American dub are actually romantic partners in the original Japanese version. She realizes Eleanor, who always used the characters' Japanese names, already knew this.
After the punishment lifts, Mina reunites with her friends at Eleanor's pool. Eleanor tells Mina that she and Margaret did not drink at a recent gathering with boys, honoring a pact the three made to experience getting drunk for the first time together. Later, Mina discovers a printed photograph Eleanor hid in her tote bag: Eleanor bare from the waist up, taken on a disposable camera. Mina realizes Eleanor planted the picture deliberately on their first day together after the kiss.
At the annual county fair, Eleanor grows overheated and asks Mina why it is so easy for her to do what Margaret wants. She tells Mina she cannot be the way Mina and Margaret are with each other. Before they can finish the conversation, Margaret returns, and Eleanor leaves abruptly. That night, Bea and her best friend Olivia reveal that during a game of Truth or Dare at a Fourth of July gathering, a boy dared Margaret to kiss Eleanor, and Margaret did. Mina is devastated. In the parking lot afterward, Margaret confronts her: She knew about Eleanor's kiss because Eleanor told her immediately, and she accuses Mina of hypocrisy for preaching honesty while keeping the biggest secret of the summer. Mina, crying, tells Margaret she is not allowed to kiss Eleanor anymore. Margaret counters that Mina never told her she had feelings for Eleanor. They ride home in wounded silence.
In the days that follow, Margaret withdraws completely. Eleanor contacts Mina directly for the first time in two months, texting that Margaret has canceled on the anniversary party for Eleanor's parents. Mina goes alone. She and Eleanor prepare together, discovering they have independently chosen offsetting black-and-white dresses. During a toast at dinner, Eleanor slides her hand beneath Mina's thigh and touches her intimately, her face perfectly composed. That night in Eleanor's bed, Mina raises the Truth or Dare kiss. Eleanor says she did not want to leave Margaret alone in the moment. In the morning, Mina returns Eleanor's photograph to a drawer with a handwritten note.
Celeste later offers quiet advice: She worries because Mina has little guile and Eleanor knows how to conceal herself. If Mina wants Eleanor to treat her a certain way, she should show Eleanor what that looks like, because Eleanor may not know. Walking through the neighborhood afterward, Mina realizes she failed to tell Margaret not because she was waiting for confirmation from Eleanor, but because she feared how loving Eleanor might change loving Margaret.
Margaret texts Mina in the middle of the night asking to be picked up from Olivia's house. She sits on the front step with mascara on her cheeks, smelling of liquor. She had told the group about the boy, only to learn that Olivia had been sleeping with him for weeks. The next morning, the two friends walk to a neighborhood marsh. Mina apologizes for not telling Margaret about Eleanor. Margaret says Mina is supposed to be the one who tells the truth. They process their mutual failures of honesty. Margaret throws her phone into the marsh. They cry, laugh, and take a picture together.
The three friends gather at Eleanor's house to get drunk together for the first time. Eleanor takes Mina outside and reveals she already got drunk alone, breaking their pact. She accuses Mina of forgiving Margaret without requiring an apology while holding Eleanor to a stricter standard. Her deeper grievance surfaces: She has spent the summer watching Margaret demonstrate oral sex on Mina and wondering if Mina will ever like her as much. Mina tells Eleanor it is not about how much but about what kind. She says she loves Eleanor and promises not to repeat it until Eleanor asks. Eleanor agrees to be Mina's girlfriend. They return to the pool, where Eleanor announces the news to Margaret, who cheers.
Days later, Eleanor invites Mina over alone. They kiss and touch each other for the first time. Neither reaches orgasm, but the encounter is tender. Afterward, Mina tells Margaret about it. Margaret admits she has never had an orgasm, and Mina helps her locate her own anatomy. They establish new rules for their trio: Mina tells Margaret to apologize to Eleanor for taking the Truth or Dare kiss for granted, and Margaret agrees. Margaret will keep both friends' secrets and decide what to share between them.
On one of the last days of summer, Mina helps her mother pull weeds in the backyard, then goes inside and tells Celeste she has a girlfriend. On the final day before school, Margaret asks what will happen if Mina and Eleanor break up. They promise to fight over Margaret but never make her testify. Eleanor posts a photo of the three of them, Mina in the middle, Eleanor licking her cheek. In a brief epilogue, the adult Mina reflects that the three now live in different cities. Margaret calls almost daily; Eleanor calls less often, usually at night, wanting to reminisce. Mina closes by describing what she offers her friends: She can be "a mirror who loves you."