What She Knew is the debut novel of British author Gilly Macmillan. First published in 2015, it was a New York Times best-selling mystery novel, nominated for both an Edgar Award and an International Thriller Writers award. Subsequent works by Macmillan feature the cast of Bristol detectives introduced in What She Knew. Titles include The Perfect Girl, Odd Child Out, I Know You Know, and The Nanny. Macmillan’s novels have been translated into more than 20 languages and have sold more than one million copies worldwide.

What She Knew is set in contemporary Bristol—a port city on England’s southwest coast. The story encompasses the nine-day investigation of the disappearance of an eight-year-old boy named Ben Finch. Ben is out for a stroll in the woods with his mother, Rachel Jenner, when he runs ahead to play on a rope swing. Rachel arrives at the swing several minutes later, and the boy is gone. A frantic search ensues with police assistance. Ben’s clothes turn up later near a pond, but the boy is still missing. The case is now escalated to an abduction, and Detective Jim Clemo spearheads the investigation.

Rachel and her ex-husband John Finch make a public statement to get the word out into the community. During the press conference, Rachel goes off-script and threatens to hunt down the kidnapper herself. Her filmed rant goes viral on the internet. The press and the public condemn Rachel’s unstable behavior, and many now believe she is responsible for her son’s disappearance.

News reporters camp out in Rachel’s front yard, escalating the stress she already feels. Their skewed coverage fuels public outrage, which causes Rachel to become the target of attacks of vandalism. Rachel’s sister and best friend rally to help her but have little success in calming Rachel’s anxiety for Ben. Jim and his team interview various suspects but make little progress in finding the boy. Rachel unsuccessfully attempts to get updates from the police, so she takes matters into her own hands. Acting on a few tenuous clues she has discovered on her own, Rachel breaks into her son’s teacher’s house and discovers that the teacher, Joanna May, was at one point holding Ben captive. The room where Ben has been is now empty.

Later that day, Ben is discovered near the spot where he disappeared. He is in a hypothermia-induced coma. Ben is rushed to the hospital and stabilized. Authorities apprehend Joanna at the local airport. At her trial, Joanna pleads guilty but never discloses any details of Ben’s captivity at her house.

In the aftermath of the crime, Rachel tries to repair Ben’s damaged psyche. He now has difficulty trusting people, just as his mother does after her ordeal with the press. Jim is riddled with guilt for not being the person who solved the crime. His chronic insomnia and panic attacks lead him to undergo psychological counseling. Rachel apologizes to readers who were expecting a more upbeat ending. “Did you want catharsis? So did I. But there was none. I’m sorry” (489).

Rachel and Jim alternately provide the perspective of the first-person narrator. Each chapter consists of events occurring during a single day of the investigation. Chapters are subdivided into segments that alternate between the two narrators. Rachel’s overwrought emotional state contrasts sharply with Jim’s detachment. Documentation relating to the case; such as internal police correspondence, blog commentary, social media posts, news articles, and transcripts of Jim’s psychological counseling sessions with notations from his therapist; is interspersed throughout the narratives

The overall tone of the book is bleak. While the plot consists of the usual activity related to a police investigation, the mental instability of the book’s two narrators also occupies a great deal of space. Rachel’s internal monologues and Jim’s conversations with his therapist are the primary vehicles used to explore themes of misplaced trust, self-doubt, and the deceptive nature of appearances.