N or M? is a novel by Agatha Christie that follows the unlikely espionage agents Tommy and Tuppence Beresford as they pursue a pair of Nazi spies who have murdered Britain's top agent.

The story is set during World War II. Tommy and Tuppence are approached by a secret agent named Grant to go undercover once more. Their mission is to seek out a man and a woman from among the colorful guests at Sans Souci, a seaside hotel. The two must try to stop a Nazi spy before he infiltrates the English government.