The ingredients of this mystery plot run all the way from the fancies of some old ladies in a home for the elderly, to dark hints at child murder, the machinations of a clever criminal gang, and the secret life of a supposedly peaceful English village.

Tommy and Tuppence Beresford, two unlikely spies, help to defend their country by trying to rout out two highly placed Fifth Columnists. When Tommy is asked by the British Intelligence Service to go and stay at the Sans Souci guest house he is astonished to see Tuppence there. The mission: to identify which of the other guests is N and which is M. - Publisher.