It is World War II, and while the RAF struggles to keep the Luftwaffe at bay, Britain faces an even more sinister threat from the enemy within - Nazis posing as ordinary citizens.
With pressure mounting, the Intelligence service appoints two unlikely spies, Tommy and Tuppence Beresford. Their mission: to seek out two highly-placed traitors, a man and a woman hiding among the colourful guests at Sans Souci, a prim seaside boarding house. But this assignment is no stroll along the promenade. After all, N and M have just murdered Britain's finest agent . . .