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 colourful 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...