N or M?

In "N or M?" by Agatha Christie, Tommy and Tuppence Beresford are recruited by British Intelligence to investigate the murder of a government agent in a seaside resort during World War II. Following cryptic final words left by the dying agent, the couple must determine which of two suspected Nazi agents known only as "N" or "M" is responsible for the crime. As they go undercover at the hotel, they uncover a web of espionage and betrayal. The novel is a classic wartime mystery featuring Christie's beloved amateur detective duo.