John Mannering, internationally known antique dealer and consultant to Scotland Yard, finds himself in prison in the new Baron story, charged with possessing stolen jewels - and with murder. The crisis begins when Mannering identifies as fakes a number of pieces of jewelry brought to him valuation. Then genuine, identical jewels are found in their place in the Quinns' strong room when the police search it, following a man's brutal murder outside Mannering's door. While the Baron awaits trial in Brixton jail, his wife and his friends must first of all persuade him not to break out and then follow his instructions in the complicated and perilous unraveling of the plat around the stolen legacy.