The novel, part of the Addicted series, follows Lily Calloway and her boyfriend, Loren "Lo" Hale. Lily, the daughter of Fizzle soda company founder Greg Calloway, has a sex addiction; Lo, the heir to Hale Co., is in recovery from an alcohol addiction. After three months apart while Lo completed rehabilitation, the couple reunites at the Princeton, New Jersey, house they will share with Lily's older sister Rose Calloway. Their relationship is now genuine rather than the fake arrangement they maintained for years to hide their addictions from their wealthy Philadelphia families. The central challenge is whether they can recover together: Lo must stay sober while helping Lily manage her sexual compulsions, even as an anonymous figure threatens to expose her secret to the world.
Lily arrives late to Lo's homecoming because of a delayed doctor's appointment. Lo notices she has been scratching her arms and biting her nails from anxiety. They have intimate sex for the first time since his departure, with Lo deliberately controlling the pace to prevent Lily from becoming compulsive. He reveals that Lily's therapist, Dr. Allison Banning, sent home a blacklist of sexual restrictions, which Lo read but decides not to share, believing Lily will fixate on what is forbidden. Moments later, Lo receives a threatening text from an unknown number claiming to know Lily's secret and planning to sell the information to tabloids. The couple panics, fearing exposure could devastate Lily's family and Fizzle.
Lo and his half-brother Ryke Meadows confront their prime suspect, Aaron Wells, a former prep school rival. Aaron denies involvement, and Ryke doubts his guilt. The group convenes at the Princeton house. Since only Lo, Ryke, Rose, and Connor Cobalt, Rose's boyfriend and heir to Cobalt Inc., know about Lily's addiction, Lily questions whether one of them leaked her secret. Connor offers to hire a private investigator. Rose voices concern that Lo may enable Lily's addiction since no one can monitor their sex life the way they could detect his drinking. Lo promises not to shut the group out.
The couple struggles to establish new routines. Lo catches Lily nearly masturbating their first night together and intervenes by spooning her with their fingers interlocked. He begins taking Antabuse, a medication that causes severe nausea if he drinks, after Ryke pressures him. At school, Lily accepts recycled exams from Sebastian Ross, Rose's manipulative best friend, who plans to undermine Rose and Connor's relationship and leverages the exams to keep Lily silent. When Lo takes a modeling job with Rose's fashion line, Calloway Couture, he overhears Rose's mother criticizing the hire. Lo quits, unwilling to damage her business.
A confrontation with Mason Nix, one of Lily's former sexual partners from Penn's soccer team, forces the couple to confront painful memories. Mason propositions Lily, referencing a night when she slept with multiple members of his team. Lo punches Mason. Shaken, Lo calls Ryke, nearly deciding to drink, but Ryke talks him down. At their first joint therapy session, Dr. Banning tells Lo that his recovery is directly linked to Lily's and advises against separating them. Both share fears: Lo worries Lily will resent his refusals, while Lily fears Lo will find someone less burdensome.
Lo secretly visits his father, Jonathan Hale, to request his birth mother's name and enlist help tracking the person behind the threatening texts. Jonathan provides the name Emily Moore and signs a contract promising not to reveal Lily's secret. Lo discovers that Lily has been hiding dozens of abusive texts. The group travels to Cancun for spring break, where a hotel booking error forces all six of them into one room. Lily's youngest sister, Daisy Calloway, a sixteen-year-old model, arrives unexpectedly. Lo and Lily notice a developing rapport between Ryke and Daisy that blurs friendship and flirtation and try to keep them apart.
During the trip, Lily makes progress. She and Lo spend a day exploring Mayan ruins, the first day Lo does not think about alcohol and Lily does not ask for sex. After Lily has an involuntary public orgasm triggered by fantasizing, she and Lo discover she can accept his comfort through touch without demanding more. Lo becomes violently ill after eating fish tacos cooked in beer and tequila, triggering his Antabuse; everyone suspects a relapse before Connor determines the cause. On the group's last night, Rose and Daisy unknowingly drink absinthe at a nightclub, and Ryke carries a hallucinating, terrified Daisy to safety.
On the flight home, Lily's sex addiction appears on national television, with Mason confirming on camera that she slept with his entire team. Daisy learns her sister's secret for the first time and accepts the truth. The group retreats to Ryke's Philadelphia apartment to avoid paparazzi while Rose handles damage control. Lo blames himself for enabling Lily, explaining that her mother neglected Lily because she was designated as Lo's future partner. The group reassures him the fault lies with their parents. Melissa, Ryke's casual girlfriend, goes on television to reveal Ryke's identity and claim Lily had sex with both brothers. When Lily and Lo meet with their parents and lawyers, Lily's mother tells her she is disgusting and has ruined the family. Jonathan presents conditions for restoring Lily's trust fund: a public apology, sessions with a parent-approved psychiatrist, and marriage to Lo within a year. He produces an engagement ring, which Lily accepts despite the coercion.
Lily begins mandatory sessions with Dr. Oliver Evans, a psychiatrist who uses electrodes to shock her when she views sexual images and repeatedly labels her deviant. Lo accompanies her, barely containing his rage. During a session Lo misses, Evans exposes himself and attempts to assault Lily; she screams for her bodyguard Garth, who intervenes, and Lily's father files a lawsuit. Meanwhile, Lo catches Lily watching pornography and masturbating. He confiscates her laptop, replaces her smartphone with a flip phone, and destroys her sex toys. Separately, Connor exposes Sebastian's years-long manipulation of Rose, and Rose permanently cuts Sebastian from her life. Lily confesses about the old exams, and Connor helps her study legitimately.
Lo travels alone to Caribou, Maine, to meet Emily Moore. She recognizes him from the news but does not invite him inside, explaining she was a teenager who became pregnant from a single encounter with Jonathan. Jonathan stopped her from having an abortion and took full custody. Lo briefly meets his half-sister Willow before leaving without resolution. In a bar parking lot afterward, Lo nearly relapses. He calls Lily, who reminds him of his childhood aspiration to be like the X-Men hero Cyclops, strong and protective, and tells him he has become that person. Lo drives to his therapist instead.
While helping Ryke pack at his mother Sara Hale's house, Lily finds a package linking Sara to the leak. Sara admits she provided the information about Lily's addiction to the press as revenge against Jonathan: Their divorce contract prevented her from publicly revealing his infidelity, but exposing Lily's secret prompted the media to uncover the affair independently. Ryke is devastated, telling his mother she has lost her only son. Lo holds his brother as Ryke breaks down.
Lo visits Aaron to apologize and delivers letters of recommendation from both Greg Calloway and Jonathan to help repair the career he once sabotaged. Aaron apologizes in return. Back at home, Lo decorates the bedroom with petals and candles and proposes to Lily with a ruby heart ring wrapped in comic book strips, replacing the coerced engagement with a genuine one. Lily says yes. Lo tells her he wants to make love, a phrase neither has used before, and when she asks what makes it different, he tells her he will show her.