Bookseller Delaney Nichols answers a mysterious invitation to meet Shelagh O'Conner, an eccentric socialite who was once suspected of murdering her boyfriend. Shelagh requests Delaney's participation in an exclusive treasure hunt, and the prize is a valuable first edition of Robert Louis Stevenson's Dr. Jekyll and Mr. Hyde. Following The Stolen Letter, this seventh outing takes a turn when several homes in Edinburgh are robbed, and Shelagh has seemingly been kidnapped. Terror mounts throughout the city as Delaney attempts to solve the mystery, while trying to evade the killer's clutches.