John Mannering (aka 'The Baron') owns Quinns, a shop in London's Mayfair, dealing in antique and precious stones. Used to solving seemingly impossible crimes, this time he is the victim. A young man seizes diamonds being shown to him, strikes 'The Baron' and makes off. Mannering is taken to hospital and is seriously ill, and whilst Scotland Yard is working hard on the case they have very little to go on.