N Or M ?

Britain is at war. While the RAF fights to hold back the Luftwaffe in the skies, the country faces a quieter danger at home: Nazi agents living among ordinary citizens, passing as neighbours.

With the pressure growing, the Intelligence service turns to two unlikely recruits, Tommy and Tuppence Beresford. Their task is to identify a man and a woman hiding among the assorted guests at Sans Souci, a seaside hotel. These two, known only as N and M, have just murdered the finest agent Britain had.

The assignment is far from a relaxed seaside break, and the pair must watch every guest closely if they are to find the enemy within.