N or M? is a work of detective fiction by Agatha Christie first published in the US by Dodd, Mead and Company in 1941 and in the UK by the Collins Crime Club in November of the same year. The US edition retailed $2.00 and the UK edition at seven shillings and sixpence (7/6).
The title is taken from a catechism in the Book of Common Prayer which asks, "What is your Christian name? Answer N. or M." The "N. or M." here stands for the Latin, "nomen vel nomina", meaning "name or names". It is an accident of typography that "nomina" came to be represented by "m".
The novel is the first to feature the mature versions of her detectives Tommy and Tuppence, whose previous appearances had been in the adventure The Secret Adversary (1922) and the short story collection Partners in Crime (1929).
After the outbreak of the Second World War and many years after they worked for British intelligence, middle-aged Tommy and Tuppence Beresford feel useless and sidelined. Then, Tommy is approached by a secret agent named Grant to go undercover once more. Grant tries to find German spies and fifth columnists but is hampered by traitors within British intelligence who follow and sabotage his work. Since Tommy has not been in the field for many years, nobody knows him, and he can investigate unofficially.
Grant explains that another British agent was recently murdered but left a cryptic message on his deathbed: "N or M. Song Susie". Grant suspects that "Song Susie" stands for Sans Souci, a hotel in (fictional) seaside Leahampton (based on Bournemouth), and "N" and "M" are known to be two top German spies, one male and one female. Tommy is to go to Sans Souci to investigate whether N, M or both are at the hotel and to figure out their identities.
Tommy is sent alone, but Tuppence decides to join him on his mission whether invited or not. Through good sleuthing, she learns where he is to be sent and arrives there before he does. Working together under separate false identities, they begin a search for the spies among the residents of the resort town.