The story opens in the affluent town of W, where two sudden shocks stun the local aristocracy on a single Saturday in June. The first shock is a daring burglary at Wardour Place, resulting in the theft of the priceless Wardour diamonds belonging to the proud heiress Constance Wardour. The second shock is the scandalous elopement of Sybil Lamotte, daughter of a wealthy resident, with John Burrill, a disreputable and divorced former mill foreman. Constance remains remarkably calm, immediately taking charge of the household and preventing a chaotic public uproar, while local officials like Mayor Soames and Constable Corliss stumble through a farcical investigation.
Seeking competent help, Constance accepts the assistance of Jasper Lamotte and his son Francis, while also consulting a mysterious newcomer, Doctor Clifford Heath. Meanwhile, the arrival of a brilliant private detective named Neil Bathurst, who drops out of an oak tree in disguise, shifts the direction of the investigation. Bathurst deduces through careful logic that the burglars were professionals who knew the premises intimately, had used minimal chloroform to subdue Constance without causing fatal harm, and had escaped by the river.
As rumors swirl about Sybil's mysterious letter and her strange choice, tensions rise within the community. Constance struggles with her growing fascination for the enigmatic Doctor Heath, even as accusations begin to fly concerning the true identity of the criminals. Suspicion falls on various figures, including the volatile Evan Lamotte, Sybil's younger brother, who battles his own demons. Private detective Belknap enters the scene with his own theories, eventually pointing a finger of suspicion at Doctor Heath, leading to a clash between Constance and the doctor.
As the narrative develops, deeper layers of self-sacrifice and hidden plots emerge, particularly regarding Evan's willingness to bear the burden of family disgrace to shield his sister. Sybil eventually returns to Mapleton with her coarse husband, facing a cold social reality. The climax builds as Bathurst returns to untangle the web of evidence, revealing that the complex web of local crimes and the diamond theft are tied to professional syndicates and personal vendettas, ultimately resolving the mystery through relentless logic and shrewd observation.
