is a work of detective fiction by Agatha Christie first published in the US by Dodd, Mead and Company in 1941[1][2] and in the UK by the Collins Crime Club in November of the same year.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.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."[5] Maurice Richardson in a short review in the 7 December 1941 issue of The Observer wrote: "Agatha Christie takes time off from Poirot and the haute cuisine of crime to write a light war-time spy thriller.N or M is [an] unknown master fifth columnist concealed in [the] person of some shabby genteel figure in a Bournemouth boarding-house ... Christie's bright young couple, now middle-aged but active as ever, are nearly trapped.