When a high-ranking British secret agent is murdered during the height of World War II, the investigation leads to a quiet seaside village where Nazi infiltrators are hiding in plain sight. Tommy and Tuppence Beresford, a middle-aged couple with a history of amateur sleuthing, are recruited by British intelligence to identify two elusive enemy operatives known only as N and M. Operating under deep cover at the Sans Souci guesthouse, the pair must navigate a web of suspicion where every guest is a potential threat. The narrative follows their efforts to maintain their own cover while systematically vetting the residents of the village to uncover the identity of the saboteurs before they can strike again.

This work is widely regarded as a quintessential example of Christie’s ability to blend the cozy mystery format with the high-stakes tension of wartime espionage. The premise suggests a focus on the domestic front, where the mundane setting of a guesthouse serves as a sharp contrast to the lethal nature of the antagonists. Readers often note the charm of the Beresfords, whose long-standing partnership provides a stable anchor amidst the pervasive paranoia of the plot. The narrative structure relies on the reader's ability to parse subtle clues, making it a classic exercise in deductive reasoning within a historical context.