During a snowbound London winter, Professor Charles Grimaud, a scholar of occult folklore and vampire superstitions, is publicly confronted at a Bloomsbury tavern by an illusionist named Pierre Fley. Fley issues an enigmatic warning regarding three brothers once buried alive in a remote Carpathian tomb, cautioning Grimaud that one of them has returned to demand retribution. Shortly thereafter, Grimaud receives an ominous telephone message announcing that an uninvited caller will visit his Warwick Square home, prompting heightened apprehension among his household, including his daughter Rosette, his secretary Anthony Hammond, and his domestic staff.

The caller arrives on a snowy evening wearing a heavy overcoat and a papier-mâché mask, and is escorted directly into Grimaud's second-floor study. Minutes later, the sound of a gunshot echoes from the room. When household members force open the door, they discover Grimaud mortally wounded from a point-blank pistol shot. The physical conditions present a dual impossibility: the study door was locked from the inside with the key still in the keyhole, the windows were bolted securely over a two-story drop into untracked snow, and the chimney was blocked with stout iron bars. Less than an hour later, in nearby Cagliostro Street, three independent observers witness the same masked figure walking in the middle of a deserted cul-de-sac; a second shot sounds in plain view, the figure falls dead, and upon inspection, no weapon is found, no shooter is visible, and the surrounding snowfield remains entirely unmarked by other footprints.

Superintendent David Hadley of Scotland Yard immediately calls upon Dr. Gideon Fell, a lexicographer and historical polymath acquainted with the victim, to untangle the contradictory physical evidence. Confronted by two violent deaths occurring under conditions that seem to suspend natural physical laws, Fell must differentiate genuine physical deception from theatrical misdirection. The inquiry demands establishing the historical identity of the three brothers, resolving how a lethal assailant could vanish from an unbroken snowfield, and containing the mounting dread before additional violence takes place.