After brilliantly resolving their last case (explored in By the Pricking of My Thumbs) Prudence is bored. She wants to set out on new adventures. She dreams of a good fairy who sends them after truly dangerous criminals and propels them into mysterious adventures. It isn′t long before the good fairy arrives in the affectionate form of Babette, Prudence′s Belgian butterfly-collecting aunt, who witnesses a horrible crime from her train carriage window: a young woman being strangled. Despite her partner Bélisaire′s scepticism, Prudence believes her aunt, and sets off to look for the corpse.