N or M?

The war has not changed Sans Souci, the prim seaside boarding house. It is still patronized by retired army men, gossipy old ladies and young people in love. But one of them is a spy, the leader of the "fifth column" of highly placed traitors who will seize power when Hitler invades England. So the irrepressible Beresfords, Tommy and Tuppence, come to live at Sans Souci ...