The Prologue is a scene removed from the main plot. The characters are unnamed, but a woman is watching a man whom she has been following for several weeks. She has seen him go through the motions of being a father and husband, but she has also observed him picking up a woman at a bar and sleeping with her in his car, going to a strip club in the middle of the day, and getting drunk with his friends.

On this particular night, the woman watches the man standing outside, waiting for someone. A woman approaches him; they speak to each other, but the observer cannot hear what they say. They go inside the woman’s house. The narrator explains that had the observer known what was about to happen, she would have stopped them; later, she learns that the man is dangerous and that something terrible happened.

Selena Murphy missed her usual train home from work and decides to take the 7:45 home instead. She works at a literary agency in New York City, and she knows that her nanny, Geneva, will get her two sons, Oliver and Stephen, ready for bed.

Selena recently moved a surveillance camera from a bedroom into the playroom because she suspects her husband, Graham, is having an affair with Geneva.