While visiting Tommy's Aunt Ada at Sunny Ridge Nursing Home, Tuppence encounters some odd residents including Mrs. Lancaster who mystifies her with talk about "your poor child" and "something behind the fireplace." When Aunt Ada dies a few weeks later, she leaves Tommy and Tuppence a painting featuring a house, which Tuppence is sure she has seen before. This realization leads her on a dangerous adventure involving a missing tombstone, diamond smuggling...

Tommy and Tuppence Beresford have just become the proud owners of an old house in an English village. Along with the property, they have inherited some worthless bric-a-brac, including a collection of antique books. While rustling through a copy of The Black Arrow, Tuppence comes upon a series of apparently random underlinings. However, when she writes down the letters, they spell out a very disturbing message: "Mary Jordan did not die naturally."...

"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 the 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 Berefords are greeted by hostile guests, a mysterious hotelier, ransacked rooms, and a disappearance....