The novel is the first to feature the mature versions of her detectives Tommy and Tuppence whose previous appearances had been in the adventure The Secret Adversary (1922) and the short story collection Partners In Crime (1929).
After the outbreak of World War 2 and many years after they worked for British intelligence, Tommy and Tuppence Beresford feel useless and sidelined. When Tommy is approached by Mr. Grant on the advice of Mr. Carter better known as Lord Easterham to go undercover once more, however, Tuppence decides to join him on his mission whether she is wanted or not.
The duo begin a search for German agents who may have infiltrated British command. Farquhar, another British agent that was following these Germans left a cryptic message on his deathbed: "N or M. Song Susie". Grant knew that "Song Susie" stood for Sans Souci, a hotel in Leahampton, and N and M were two German spies, one male and one female. Tommy is to go to Sans Souci to investigate whether N, M or both are at the hotel and to figure out their identities.
The plot of the book is filled with many plot-twists and an action-packed denouement.