N or M?

This atmospheric story, set during World War Two, finds Tommy and Tuppence doing what they do best '“ having an adventure. These unlikely Intelligence Service spies decide to help Queen and country by tracking down two ruthless traitors.The only clue to the traitors' identities is a government agent's dying words that lead them to the Sans Souci boarding house, where it'll take some extremely subtle detection work to establish which of the guests are the treacherous N and M.