"N or M Song Susie" were the last words uttered by a government agent. Now Tommy Beresford is asked by the British Intelligence Service to try to smoke out an enemy agent who is believed to be living at the Sans Souci Hotel in the seaside town of Leahampton. Refusing to be left behind, Tuppence joins him. Once inside the Sans Souci, the intrepid Beresfords are greeted by hostile guests, a mysterious hotelier, ransacked rooms, and a disappearance. And when Tommy himself disappears, Tuppence realizes that the stakes couldn't possibly be higher. It is World War II, and while the RAF struggles to keep the Luftwaffe at bay, Tommy and Tuppence face an extremely sinister threat from "the enemy within."