The following version of the novel was used to create this study guide: Disclaimer. Renée Knight. New York: HarperCollins, 2015.

Catherine Ravenscroft is a documentary film maker with a lawyer husband, Robert, and an unmotivated young adult son, Nicholas. One day soon after moving into a new house Catherine mysteriously finds a book called "The Perfect Stranger" in her home. After reading part of the book Catherine is terrified because she is sure she is the main character in the book. The book details an traumatic event that happened to Catherine 20 years prior and ends with the main character (Catherine's proxy) being killed by being pushed in front of a train.

Stephen Brigstocke is a retired teacher. He was forced to retire after writing a cruel comment on a young student's paper. He was also previously fired from a job for becoming obsessed with one of his students. Stephen lost his wife, Nancy, several years prior to the start of the narrative and lost his son, Jonathan, even before that. Stephen finally decides to clear out his wife's belongings and he finds a envelope full of lewd pictures of Catherine. It is revealed Stephen's son Jonathan had an affair with Catherine 20 years ago in Spain. One day on the beach Catherine's then-young son Nicholas drifted out to sea on an inflatable toy and Jonathan died saving him. Stephen then finds a manuscript written by Nancy unbeknownst to Stephen years prior detailing the incident. Stephen decides to self-publish the book and use it to torment and eventually kill Catherine and her family as revenge, as he holds Catherine responsible for the death of his son.

Catherine keeps the book a secret from her family while she figures out what to do. Stephen goes to Nicholas's work and gives him a copy and leaves a copy for Robert at his office. Catherine reveals to Robert the book is about her which makes Robert furious, thinking his wife had an affair and almost let their son die. He kicks her out of the house and does not tell Nicholas the truth. Catherine deduces Stephen Brigstocke wrote the book and attempts to reason with him but to no avail. Stephen's mental health deteriorates into an unreasonable state, believing Nancy speaks to him and is telling him what to do. Eventually, Stephen creates a fake Facebook profile for his son Jonathan. Through this profile Stephen befriends Nicholas and reveals his mother had an affair and almost let him drown. This prompts Nicholas to overdose on drugs, putting himself in a coma.

While Nicholas is recovering, Robert refuses to speak with Catherine. She finally finds Stephen at his home and reveals the events detailed in "The Perfect Stranger" are inaccurate. Jonathan brutally raped Catherine while Nicholas laid in the next room. He had been stalking her the previous few days and propositioned her at a bar, which Catherine rebuffed. The day after the rape Catherine was so distraught and distracted she did not notice Nicholas wandering out to sea. Jonathan was also on the beach and ran to save Nicholas, drowning in the process. Stephen is shocked but also believes Catherine, as Jonathan had shown violent tendencies before. Stephen tells Robert the truth which prompts Robert to make amends with Catherine. Catherine is angry that Robert seems relieved that Catherine was raped rather than her having an affair. The book ends with the state of their marriage in question.

When Nicholas wakes up from his coma Catherine tells him the truth about everything and she takes care of him until he recovers, reinvigorating their relationship. Stephen changes his will to leave his house to Catherine, which are then transferred to her after Stephen dies, implied to be from suicide. The book ends with Catherine still unsure about her marriage but committed to being a good mother to Nicholas for the first time in his life.