Tommy and Tuppence Beresford are recruited by British intelligence to identify a dangerous Nazi spy hiding in a seaside boarding house during the height of World War II. As the couple assumes new identities to infiltrate the suspicious environment, they must navigate a web of paranoia and shifting loyalties while their own lives remain at risk. The narrative follows their investigation as they attempt to unmask the elusive agents known only as 'N' or 'M' before a catastrophic attack occurs. The story utilizes a third-person perspective to track the couple's domestic cover and their clandestine efforts to secure the nation's safety.

This novel is widely regarded as a departure from Christie's traditional village mysteries, leaning heavily into the spy thriller genre of the 1940s. Readers often note that the charm of the Beresfords provides a lighter, more conversational tone compared to the more clinical investigations of Hercule Poirot. The historical context of the war adds a layer of urgency and tension that distinguishes the plot from her earlier works. While some critics point to the dated nature of the espionage elements, the book remains a staple for those interested in the evolution of the mystery genre during the mid-twentieth century.