M (N or M?)

In the novel N or M?, M is the code name given by British Intelligence to an unknown female German spy. British intelligence had come across her and her colleague codenamed N operating in other countries earlier on. They had a mission to organise Fifth Column activities in foreign countries. At the beginning of the war British intelligence had intercepted a coded message (presumably from German intelligence), which said: "Suggest N or M for England. Full powers".

The challenge for Tommy and Tuppence in the story is to uncover who M and N are.