Grey Mask

First published in 1929, the first book in the series featuring the private investigator Miss Maud Silver.

Charles Moray leaves home after being jilted by Margaret Langton on the eve of their wedding and returns four years later to find his home open and a strange meeting taking place. Watching through a childhood hideout, he sees a man in a grey mask talking to several different people - calling each by a number. When Charles hears them discussing "removing" a girl if a "certificate" is found, he is shocked. He is even more shocked when he recognizes one of the agents - his former fiancee.
When he reads about Margot Standing, whose millionaire father dies leaving her inheritance in the balance because of a missing marriage certificate, he puts two and two together and approaches Miss Silver, a private investigator with a high success rate in missing jewelry. He is skeptical until Miss Silver astutely guesses he will not go to the police because of his former love.