Amidst the uncertainty of World War II, middle-aged amateur sleuths Tommy and Tuppence Beresford are recruited by British Intelligence to uncover a pair of dangerous Nazi saboteurs hiding in plain sight. Tasked with infiltrating the Sans Souci seaside hotel, the duo must navigate a web of suspicion where any guest could be a lethal enemy agent. They operate under the constant pressure of knowing that their predecessors were murdered, forcing them to rely on their wits rather than formal training. The narrative unfolds through a third-person perspective that emphasizes the domestic tension of wartime Britain and the psychological games played by the protagonists.

This work is widely regarded as a notable entry in the Tommy and Tuppence series, showcasing Christie's ability to pivot from traditional country house mysteries to wartime espionage. Readers often highlight the charm of the central couple, whose long-standing partnership provides a stable anchor against the backdrop of global conflict. The setting of the seaside hotel is frequently cited as a clever device that allows for a diverse cast of suspects to be confined in a single location. While it lacks the iconic presence of Poirot or Marple, the novel is praised for its suspenseful atmosphere and the effective integration of historical anxiety into the detective genre.