After closing their detective agency, Tommy and Tuppence Beresford move to the resort town of Hollowquay for their retirement - which is quickly postponed. Tuppence discovers a children's book signed with a child's name, "Alexander Parkinson." Scrawled inside is a cryptic message: "Mary Jordan did not die naturally. It was one of us. I think I know which one." Tommy joins her in trying to solve the old mystery, but danger looms as they get closer to finding what really happened in this town years ago.
