Recovering their balance after their adventures in Lincoln and the discovery of their secret sister, Aventurine Morrow and her twin Micheline are holed up together in Hay-on-Wye. There, they receive several coded messages indicating the possibility that Mick's husband, Shep, lost at sea, might actually have died by suicide. When her twin disappears, Avi must join forces with the always-suspicious policeman Dominic Burroughs to decipher those clues and attempt to find Mick. They seek the help of former SOE agent Genevieve Smithson, but that wily old spy might know more than she's letting on. The chase takes them to London, where Burroughs and Aventurine are joined by Micheline's son, Paul, and his partner, Lance. Someone is obviously watching them, however, as the coded clues keep appearing. As they all investigate—and disagree on the meaning of the clues—everyone falls away from Aventurine until she finds herself in the North Yorkshire village of Whitby, facing surprising danger—alone.