When one of the Robthorne twins commits suicide, there is a question over which one it is and whether it was, in fact, suicide or murder. From the Dr. Priestley, well-known crime investigator, is called in to solve the mysterious death of Mr. Robthorne, who has been found shot in the greenhouse of his country estate. A chain of damning evidence that Dr. Priestley pieces so successfully together forms one of the finest examples of crime detection that Mr. John Rhode has yet produced.