A series of minor thefts at Vere House prompts the owners to call in their old friend, John Mannering, to investigate. Whilst Mannering is doing so, the jewels and the famous Deverall necklace belonging to Lady Usk, a guest of the Vere's, are stolen. Reluctantly, the police are brought in and much to Mannering's disquiet, Scotland Yard send in Inspector Bristow, one of the few who suspect Mannering to be 'The Baron' — the cleverest jewel thief in the country. Mannering must prove his innocence when he discovers someone has planted the stolen Deverall necklace in his room . . .