During the height of World War II, two aging amateur sleuths are recruited by British Intelligence to identify and neutralize a pair of dangerous Nazi spies embedded within a seaside hotel. Tommy and Tuppence Beresford, now middle-aged and feeling sidelined by the war effort, accept the clandestine mission to infiltrate the Sans Souci hotel. They must navigate a web of suspicion where every guest is a potential enemy, all while operating under the constant threat of discovery by the very agents they are tasked to expose. The narrative unfolds as a high-stakes game of cat and mouse, testing the couple's ingenuity and their long-standing partnership against a backdrop of national paranoia.

This novel is widely regarded as a departure from Christie's traditional village mysteries, shifting the focus toward the geopolitical anxieties of the wartime era. Readers often highlight the charm of the Beresfords' relationship, which provides a grounded, human element amidst the darker themes of espionage and betrayal. The setting of the seaside hotel serves as an effective microcosm for the suspicion and paranoia that permeated British society during the conflict. While some critics note that the spy-thriller elements are less intricate than her Hercule Poirot puzzles, the book remains a significant entry in the author's bibliography for its historical context and character development.