The Necklace Affair London 1817
Captain Lacey agrees to help a society matron discover what has become of her diamond necklace and to clear her maid, who has been arrested for its theft. Lacey soon realizes that the problem is not so simple—the question involves scandal and past secrets, going back to the French Revolution and its aftermath—plus he finds himself competing with the underworld criminal James Denis for the necklace's retrieval.
This is a 25,000-word (ten-chapter) novella. The events in it occur between end of The Sudbury School Murders and the beginning of A Body in Berkeley Square.