Tommy and Tuppence Beresford, now in their seventies, retire to an old house called The Laurels in a quiet seaside village, hoping to settle into a peaceful final chapter together. The property comes cluttered with the previous owners' belongings, including shelves of dusty children's books that delight Tuppence with memories of her own girlhood reading. While sorting through one battered volume, she notices words underlined in faded ink and pieces together a hidden message claiming that a girl named Mary Jordan, dead for decades, did not die naturally. The cryptic line points back to events in the village before the First World War, to old rumours of espionage, and to a death that some long suspected was no accident. With little better to occupy them, the Beresfords decide to dig into the half-buried history for the sheer pleasure of solving a puzzle. What begins as a gentle diversion among gossiping neighbours, faded photographs, and the recollections of elderly villagers gradually takes on weight, and the past proves less harmless than it first appeared. Aided by their devoted factotum Albert and their fiercely loyal Manchester Terrier, Hannibal, the couple follow a trail that draws in shadowy figures from the world of secret intelligence. As their inquiries deepen, someone decides the old secret is worth protecting, and the comfortable retirement the Beresfords imagined begins to feel decidedly precarious.