John Mannering, retired jewel thief known as 'The Baron', now runs Quinns, an antique shop in London's Mayfair. He is offered a superb set of diamonds which turn out to have been stolen in a daring robbery from the Louvre in Paris. Mannering is now faces a dangerous dilemma, because the jewels may lead him to a murderer, with a further possible death to be prevented; and at the same time he is tempted to return to his old profession, at least to the extent of handling the diamonds. His decision leads to a daring and dangerous adventure, in which he is kidnapped and threatened with death, and which has all the hallmarks of being one of his most exciting escapades.