A shot rang out from Quinns, a shop in Mayfair dealing in antiques and precious stones. John Mannering - alias the Baron - had been showing some diamonds to an amiable-looking young man. However, looks can be deceiving, as the young man seized the jewels and shot the Baron in the head before fleeing. Now Mannering was lying gravely ill in Westminster Hospital, undergoing an emergency operation. Scotland Yard were on the case, but did they didn't have much to go on....