N or M?

"It is World War II, and while the RAF struggles to keep the Luftwaffe at bay, Britain faces an even more sinister threat from 'the enemy within'--Nazis posing as ordinary citizens. With pressure mounting, the intelligence service appoints two unlikely spies, Tommy and Tuppence Beresford. Their mission: to seek out a man and a woman from among the colorful guests at Sans Souci, a seaside hotel. But this assignment is no stroll along the promenade--N and M have just murdered Britain's finest agent and no one at all can be trusted..."--Page 4 of cover.