Jessica shows up to a psychology study pretending to be a client named Taylor and is enrolled as Subject 52.

Dr. Shields suspends her broader study to make Jessica the sole subject of a new real-world experiment.

Jessica meets Dr. Shields in person at her office and agrees to join an expanded morality study for substantial pay.

Jessica breaks down while confessing to Dr. Shields that she locked her sister Becky in a room on the day of the accident.

Dr. Shields deliberately triggers Thomas's nut allergy during a dinner to get time alone with his phone.

Dr. Shields sends Jessica to a hotel bar to flirt with a married man, who firmly rejects her advances.

Jessica unknowingly meets Thomas outside an art museum during a street accident and exchanges contact information with him.

Jessica discovers at a diner that the man she was sent to meet is Thomas, the same man she had slept with.

Thomas warns Jessica after their diner meeting that Dr. Shields is dangerous and preys on young women.

Thomas leads Jessica to a conservatory garden and reveals that a previous study subject, April, died there by suicide.

Jessica sneaks into Dr. Shields's home while Thomas keeps her distracted and photographs both Jessica's and April's files.

Jessica visits April's mother while posing as an old college friend to gather information about April's death.

Dr. Shields confronts both Thomas and Jessica at a Christmas dinner and threatens each of them with separate blackmail evidence.

Jessica accuses Dr. Shields of deliberately giving April a fatal dose of pills, forcing a confession from her.

Thomas rejects Dr. Shields completely after she admits to causing April's death, leaving her utterly alone.

Jessica Farris sneaks into a psychology study by impersonating another participant and quickly becomes the sole focus of the mysterious Dr. Shields. Drawn into an elaborate real-world experiment about marital fidelity, Jessica finds herself entangled with Dr. Shields's estranged husband Thomas in ways neither anticipated. As Jessica uncovers the truth about a previous subject named April who died by suicide, she and Thomas work together to expose Dr. Shields's crimes. The story builds to a dramatic Christmas dinner where Dr. Shields's darkest secret is finally forced into the open. Abandoned by Thomas after her confession, Dr. Shields ends her own life, leaving Jessica to walk away changed — but not too changed to spare Thomas from one final act of blackmail.

The central thriller from study infiltration to deadly confrontation.

Jessica secretly listens to a voicemail about a paid psychology study while finishing a makeup appointment.

Dr. Shields notes during Jessica's first session that she hesitates longest when asked if she has ever hurt someone she loves.

Jessica visits her parents and learns her father lost his job and the family can no longer afford their Florida trip.

Jessica researches Dr. Shields online and is surprised to discover the therapist is a woman, not a man.

Jessica stalks Dr. Shields at her university lecture and follows her to a romantic restaurant to observe her.

Dr. Shields goes to Thomas's office building at night and watches a woman enter just as his office blinds close.

Dr. Shields assigns Jessica to perform free makeup sessions for women pulled from Thomas's recent call log.

Jessica is left alone in a dangerous situation at Tiffani's apartment when Dr. Shields suddenly hangs up mid-session.

Dr. Shields's assistant Ben informs Jessica that the original psychology study was formally terminated weeks earlier.

Dr. Shields secretly pays for Jessica's family's Florida vacation as a way to isolate and bind Jessica to the experiment.

Jessica follows Thomas and Dr. Shields to a restaurant and watches them appear affectionate, making her doubt Thomas's loyalty.

Jessica visits the clothing store owner Thomas claimed to have had an affair with, and the woman flatly denies it.

Dr. Shields reports Jessica to her employer for the free services, getting her fired, and tells Noah damaging lies about her.

Jessica confesses to her parents on Christmas Day that she locked Becky in her room the day of the accident.

Jessica meets Thomas one final time after Dr. Shields's death and blackmails him into paying her a monthly sum.

Before Jessica ever steps foot in the study, small impulsive decisions set an irreversible chain of events in motion. As her involvement with Dr. Shields deepens, Jessica peels back layer after layer of deception — researching her therapist, receiving warnings from a fired assistant, and surviving dangerous assignments that push her to her limits. Her personal life frays under the pressure as Noah is driven away, her job is taken, and her family is manipulated without her knowledge. Key discoveries along the way challenge the official account of Thomas and April, forcing Jessica to question everyone around her. Only by rebuilding trust with her parents and leveraging what she knows can Jessica reclaim her life — though even then, she exits the story with one last morally ambiguous act.

The smaller moments that reveal character and steadily build dread.

Jessica answers yes when asked whether a person who cheats once will always cheat again, planting doubt in Dr. Shields about Thomas.

Dr. Shields decides to use Jessica as a variable to secretly test whether Thomas will submit to temptation with another woman.

Dr. Shields gifts Thomas a glass falcon sculpture, an object symbolizing both affection and her readiness to strike.

Jessica and Thomas sleep together after their chance meeting at the accident scene, unaware of each other's connection to Dr. Shields.

Thomas sends Jessica a wedding photograph proving he is Dr. Shields's husband and that she constructed the entire setup.

Dr. Shields leaves a bowl of chicken soup at Jessica's apartment door as a veiled threat after Jessica lies about being sick.

Jessica presses Dr. Shields with half-truths during a meeting until Dr. Shields admits Thomas is her husband and reveals her true motive.

Dr. Shields reveals in her own narration that she walked April to the garden, gave her pills, and left her there alone.

Jessica goes to Noah's apartment to apologize for lying and tells him her real name for the first time.

Jessica discovers from April's file that April met Thomas before joining the study, suggesting she entered to get closer to him.

Dr. Shields realizes she was so focused on monitoring Thomas that she completely failed to notice Jessica becoming a genuine threat.

Dr. Shields spends Christmas Eve with her family and privately recalls locking her teenage sister out of the house the night she died.

Jessica contacts the private investigator hired by April's family and schedules emails compiling evidence against Dr. Shields to send automatically.

Jessica buys a voice-recording watch before dinner and arrives at Dr. Shields's home to find Thomas already seated in the library.

Dr. Shields writes a final confession protecting Thomas's reputation and ends her own life the same way April did.

Beneath the surface of the main plot lies a series of pivotal revelations that reframe the entire story. A single answer Jessica gives in an early session unknowingly launches Dr. Shields's obsessive scheme, while hidden truths about April, Thomas, and Dr. Shields herself are uncovered one by one. Key confrontations force characters to drop their facades, culminating in Dr. Shields's chilling private confession about how April really died. Dr. Shields's own buried past — a teenage sister locked out of the house and killed — mirrors Jessica's guilt and explains her compulsive need to possess younger women. In the novel's final act, Jessica outmaneuvers Dr. Shields by recording their last meeting and pre-scheduling evidence to be released, surviving the same trap that destroyed April.