John Mannering (aka 'The Baron') is an ex-cracksman, who is not only now 'straight', but can also act when needed in solving crimes. His wife has painted and exhibited and the Summer Exhibition of the Royal Academy what may critics believe to be the finest work that year. She learns from her contacts that Quinns, a curio shop in London's Mayfair is for sale, and urges Mannering to buy it. His attempt to do so, however, is shrouded in mystery, and the police start to wonder if once again he has turned his hand to being a master jewel thief.