James Caplon Answell visits the London residence of Avory Hume, a former diplomat and prominent sporting authority, to formally seek permission to marry Hume's daughter, Mary. Answell is an athletic young man of unblemished reputation with no known enmity toward the Hume family. Hume receives him into his private ground-floor study, an interior designed as an impregnable security room containing a collection of historical arms and an extensive cellar. After exchanging brief introductory remarks, Hume pours drinks for both men and fastens the room's access points.

Moments after consuming his whiskey, Answell experiences severe vertigo and loses consciousness, having ingested a fast-acting sedative. Upon waking, he finds Hume lying dead before the fireplace, pierced through the heart with an arrow taken from a pair of antique crossbows mounted upon the study wall. The mechanical security of the chamber is absolute: the heavy oak door is secured from within by two solid steel bolts, the windows are sealed by bolted interior iron shutters, and the hearth chimney is blocked by a permanent iron grate. Only Answell and the deceased occupy the room, and Answell's fingerprints alone are recovered from the shaft of the crossbow bolt.

Arrested on capital charges, Answell is committed for trial at the Old Bailey, where the prosecution constructs an apparently unassailable circumstantial case under Senior Treasury Counsel Sir Walter Storm. With the defendant facing execution, defense solicitor Arthur Bailey enlists Sir Henry Merrivale, the cynical barrister and former head of the Secret Service's counter-espionage division. Taking his place at the defense table, Merrivale mounts a high-stakes courtroom defense founded upon a specific physical premise: that every locked chamber contains an overlooked opening—a 'Judas window'—through which an external assailant could strike without breaching the room's physical boundary.