A country-house setting centers on preparations for a secretive masked ball and the arrival of a recently appointed Chief Constable who is an acquaintance of the household. A theft during the festivities escalates into murder, and a series of puzzling discoveries — a body in the lake, a death in the family museum, and a mysterious sword — prompt methodical inquiries. The narrative follows investigations, chases through woods, subterranean explorations, and the uncovering of cunning deceptions until a logical solution emerges. Themes include disguised identities, the contrast between appearance and fact, and the use of deduction to expose hidden motives.