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. After all N and M have just murdered Britain's finest agent…