The novel alternates between the present-day investigation of a private detective and flashbacks revealing a hidden partnership between two killers who have been orchestrating deaths since adolescence.
Henry Kimball, a private investigator in Cambridge, Massachusetts, receives a visit from Joan Whalen, a former student from his one year as a senior honors English teacher at Dartford-Middleham (DM) High School. Joan hires him to confirm that her husband, Richard Whalen, who owns a real estate company called Blackburn Properties, is having an affair with his office manager, Pam O'Neil. She says she already knows about the affair but wants proof. Kimball agrees, unsettled by seeing Joan again. His time at DM ended in trauma: A quiet student named James Pursall pulled a gun during class, shot and killed a classmate named Madison Brown, and then killed himself. Kimball froze during the incident, unable to act, and the guilt drove him from teaching into police work and eventually into private investigation.
Intercut with Kimball's investigation are flashbacks to the summer of 2000, when 15-year-old Joan vacationed at the Windward Resort in Kennewick, Maine. An older teenager named Duane Wozniak lured her to a beach bonfire and assaulted her. The next day she met Richard Seddon, a socially isolated classmate who turned out to be Duane's cousin. Richard told Joan that Duane was the worst person he had ever met, and their conversations quickly shifted to fantasies about killing him. Joan suggested luring Duane to the end of a stone jetty on a stormy night and pushing him into the ocean. They decided to become "secret friends" whose connection no one would ever know about, an arrangement that became the foundation of their criminal partnership.
In the present, Kimball stakes out Blackburn Properties and follows Pam to a restaurant near her apartment. Posing as a poet named Henry Dickey, he befriends her over cocktails. Pam describes her love life as "a complicated and stupid relationship" that is "more of a threesome than a twosome" (57) but refuses to name the man. Over multiple evenings they grow closer and eventually sleep together. Kimball feels guilty but hopes the affair between Pam and Richard Whalen will soon yield the evidence Joan needs.
The flashbacks reveal Joan and Richard Seddon executing their plan. On the chosen night, Joan guided Duane along the dark, wave-battered jetty and positioned him on a lower rock. She kissed him, then shoved him backward into Richard Seddon, who was hiding behind a fallen granite block. Richard grabbed Duane and pivoted him toward the edge. Duane's head struck a rock, and a massive wave swept him into the ocean.
Back in the present, Kimball follows both Richard Whalen and Pam to a secluded house in Bingham listed by Blackburn Properties. Walking up the driveway, he hears three gunshots, calls 911, and enters to find Pam shot dead on one sofa and Richard Whalen dead on the other with a gun in his hand and a bullet wound to his temple. The scene is staged to look like a murder-suicide, and police treat it as exactly that.
Joan sends Kimball an overpaid check expressing shock. But Kimball grows suspicious when he finds an old classroom exercise in which students wrote about where they would be in 10 years. Joan's entry describes being "filthy rich because my first husband will have died in mysterious circumstances" (127), with "Richard Gere" providing her alibi. The eerie parallel to her actual life sends Kimball to Monk's House, Lily Kintner's family home in Connecticut, to consult the woman who once nearly killed him but has since become an unlikely confidante. Lily concludes that Joan never acts alone and always recruits a partner to do her killing.
A chapter narrated from Richard Seddon's perspective reveals the full scope of the partnership. Five years before the murders, Joan sought him out and told him she had married Richard Whalen, that the marriage was miserable, and that she had engineered Pam's affair by encouraging Pam to sleep with her husband. She asked Richard Seddon to kill both victims in the empty house they used for trysts, staging the scene as a murder-suicide, and planned to hire Kimball as a witness who would discover the bodies. The same chapter reveals that Joan previously orchestrated the school shooting by having Richard Seddon manipulate James Pursall into killing Madison Brown, framing the attack as a mutual pact that Richard never intended to honor.
Kimball traces the connection between Joan and Richard Seddon through a poem by Joan's sister, Elizabeth Grieve. Titled "Tides," the poem describes the speaker's sister swimming past the breakers with a boy who "came back all alone" (175-176), the sister having "just turned / the tender age of murderers" (175-176). Kimball confirms that both Joan and Richard Seddon were at the resort the summer Duane drowned and locates Richard in Fairview, Massachusetts. He confronts Richard at his workplace under the guise of reinvestigating the drowning and installs a GPS tracker on his car. Alarmed, Richard decides to eliminate Kimball on his own and drives to Kimball's office with a backpack containing a homemade bomb. During a tense exchange, he leaves the backpack behind. Kimball picks it up, sees Richard at the top of the stairs pressing a remote detonator, and hurls the bag at him before diving into his office. The explosion kills Richard Seddon instantly, while Kimball survives with a subdural hemorrhage.
Lily learns of the bombing from the news and drives to Cambridge, where she searches Kimball's apartment and finds a notebook summarizing his conclusions about Joan and Richard Seddon. She transforms her appearance, dyeing her hair platinum blond and dressing in punk clothing. Using a burner phone and the alias "Addie Logan," she contacts Joan, claiming to be Richard Seddon's friend who knows about their crimes. Joan denies everything, but Lily plants a hook: She says she knows how to get rid of Kimball for good.
Joan calls back, and they meet at the Fairview Library. Lily fabricates a murder confession to gain Joan's trust, and Joan drops her denials. Lily proposes smothering Kimball with a pillow, but Joan suggests a more untraceable method: pushing a sharpened piece of piano wire through the corner of his eye into the brain, which would mimic a natural hemorrhage. They agree that Joan will call in a fake bomb threat to the hospital as a diversion while Lily enters Kimball's room.
Joan makes the diversion call, but Lily never goes to the hospital. Instead, she breaks into Joan's house and hides upstairs. She has spiked Joan's wine and vitaminwater with chloral hydrate, a powerful sedative. Joan drinks the drugged beverages and falls into a deep sleep. At midnight, Lily stands over her with the sharpened piano wire, the very instrument Joan prescribed for killing Kimball.
A brief interlude flashes back to the resort library the night after Duane's death, where Joan and Richard Seddon declared themselves "secretly married" (289), exchanging vows sealed not by ceremony but by what they had done together. This scene captures the emotional core of a bond that drove decades of killing.
Weeks later, Kimball arrives at Monk's House, thinner and limping, with significant memory gaps from the explosion. Joan has been found dead from an apparent brain hemorrhage, with no foul play suspected. Kimball notes that Joan's death seems suspiciously convenient but does not press the point. He tells Lily he accepts that his love for her does not require reciprocation. Lily replies that romantic love is "the most destructive force on earth" (302), then kisses him lightly before watching him drive away.