Ethan hosts the annual Mastery Seminars, a week-long gathering of high-profile figures at a California resort, with all proceeds benefiting his Anti-Child-Labor Initiative. Jonah attends the gathering as Ethan’s guest. While Ethan handles his hosting duties, Jonah wanders toward a seminar on reinvention, drawn in by music drifting from a ballroom. Inside, he discovers Barry Claimes performing “The Selfish Shellfish,” one of the songs Jonah wrote as a child before Barry drugged him and stole his musical ideas. Watching Barry perform to an admiring crowd and then narrate a tidy, self-congratulatory story of personal reinvention, Jonah is overwhelmed with grief and fury.

Jonah follows Barry to his hotel room and forces his way inside. The two men struggle physically, and Jonah strikes Barry across the face with his own banjo, breaking his nose and drawing blood. Jonah then calmly washes up, takes stock of Barry’s modest toiletries, and leaves with the banjo in hand, confident Barry will not call security.

Later, meeting Ethan at a vineyard, Jonah is visibly shaken. He finally discloses to Ethan that his music was stolen when he was young, and that this drove him away from a life in music. Ethan gently challenges him, suggesting that the theft took only some of his music, not all of it, and encourages Jonah to simply play for himself. That evening, alone in his suite, Jonah picks up the stolen banjo and plays until dinner.

Life at Spirit-in-the-Woods proceeds with minor incidents. Noelle is sent home after she experiences bulimia and releases the llamas from their pen in protest. There are also reports of unfamiliar men near the camp’s wooded edges. One afternoon, while investigating a reported trespasser, Jules encounters Goodman, who has returned from Iceland and is camping nearby with a young stranger named Martin. Goodman claims he has been drawn by fantasies of buying cheap rural property and hints he hopes Ash and Ethan might financially support such a move. When Dennis arrives, he forcefully orders Goodman to leave and escorts Martin back to the local psychiatric hospital he walked out of for further care.

Jules immediately calls Ash, not realizing Ash is on speakerphone with Ethan. The accidental disclosure of Goodman’s presence sets off a catastrophic argument between Ash and Ethan in their Prague hotel room. By the next morning they have separated. Ash retreats to their Colorado ranch with Mo. She assures Jules she holds no blame toward her, but remains distant and unavailable.

With Ash and Ethan’s separation weighing heavily on her, Jules struggles to be fully present at camp. When the Wunderlichs visit at summer’s end and offer a five-year directorship contract, Dennis is enthusiastic, but Jules hesitates. That evening, Dennis confronts Jules directly, accusing her of having idealized a return to camp life and of remaining more emotionally invested in her friends than in their own marriage. He argues that what made the camp extraordinary when Jules was young was simply being young, not the place itself. Jules cannot fully refute this. They decline the offer and return to New York.

Back in New York that autumn, Jules is unable to rebuild her therapy practice, while Dennis returns to the Chinatown clinic and earns a raise to compensate for Jules’s unemployment. Their marriage has settled into a strained but functional truce. Jules visits Lois in Underhill to help sort through the family home before she moves to a smaller condo. Jules has a candid exchange with her sister, Ellen, who clarifies that her past coldness toward Jules was simply personality, rather than jealousy.

In December, Ethan reaches out and invites Jules to dinner at his studio. She arrives to find Mo briefly present, now 19, socially awkward but learning to play banjo on Skype with Jonah. After Mo leaves, Ethan and Jules talk honestly about the Goodman situation, the decline of his marriage to Ash, and his complicated feelings about being a father to Mo. The conversation shifts to Cathy, whom Ethan has quietly remained in contact with since her public scandal in 2001. Both agree Goodman almost certainly assaulted her.

Ethan then attempts to kiss Jules, and she briefly allows it before pulling back, feeling certain it is wrong. She thinks about Dennis and chooses to go home. Before Jules leaves, Ethan reveals he has been diagnosed with melanoma that has spread to his lymph nodes. He has kept this from Ash and has undergone two ineffective rounds of chemotherapy. Jules agrees to accompany him to appointments and insists he must tell Ash. Back home, she tells Dennis about Ethan’s cancer. Dennis holds her, and their marriage re-stabilizes around the weight of this news.

That winter, Ethan and Ash reconcile. The Interestings, save for Cathy and Goodman, gather twice for quiet dinners, though everyone is subdued. Jonah, who is playing music again with a small group of musicians in Brooklyn, continues teaching Mo banjo online. Jules, meanwhile, has found meaningful work running therapy groups for adolescents at a mental health center in northern Manhattan.

Ethan’s health worsens. He and Ash travel to a clinic in Switzerland for an experimental treatment, but Ethan quits early, sickened by the drugs. Back in New York, Ethan’s family withdraws from him. Mo, barricaded in his room, plays banjo. Ethan enters his son’s room and listens as Mo performs a recognizable song for him; Ethan cries.

One afternoon, Ash calls Jules to say Ethan has died of a massive heart attack, which was likely a consequence of his accumulated treatments. Jules absorbs the news slowly, thinking of her father’s death years before. She returns to her therapy group and tells the teenagers her friend has died; they gather around her. That evening, Jules and Dennis go to Charles Street and stay through the night, helping the family. In the weeks that follow, the affairs of Ethan’s substantial estate remain a matter of quiet speculation among his friends.

A month after Ethan’s death, Ash sends Jules a package: an old storyboard Ethan drew when they were both around 15, depicting the two of them at camp. Jules recognizes the moment when she rejected Ethan’s feelings for her, the two of them walking together anyway, Ethan privately registering both heartbreak and happiness. Jules places the drawings in a chest alongside her old camp yearbooks. The novel closes with her reflecting that life has been a recurring sequence of longing, envy, failure, and connection, which is visually represented by the aerial photograph of Ethan’s, Jules’s, and Goodman’s bodies making contact with one another.