Nuclear physicist Gilbert Worthing is abducted from a Norfolk RAF base along with secrets vital to national security. Carter gets Tommy to link up with agent Harrison but he is murdered by a hit-woman who also pursues Tuppence. Harrison's dying words concern N and M, two Russian spies, and lead Tommy to the Sans Souci hotel on the Norfolk coast where Gilbert was staying, to unmask the traitors. He is followed by Tuppence and together they meet a group of likely suspects including the glamorous Mrs Sprot, cocky young Carl Denim and two military types, Commander Haydock and Major Khan, the latter challenging Tuppence.