In this story, Dr. Watson arrives at 221B Baker Street where Boy Billy shows him a wax effigy of Holmes placed near a curtained window in the sitting room. The effigy produces a shadow on the curtain that, when viewed from outside, is the unmistakable profile of Sherlock Holmes. Using this visual trick Holmes aims to give a perfect target to a would be murderer with a rifle. Holmes names his murderer as Count Negretto Sylvius, the diamond thief he has been following in disguises. He gives the criminal's address to Watson, then sends the doctor out the back for the police. Surprised by the Count, Holmes offers him and his helper, boxer Sam Merton, freedom if they give up the jewel, or jail if not.
He invites them to discuss the deal while he plays violin in the next room. When the Count decides to double-cross Holmes and takes the stone from his secret pocket to show Sam in window light, the detective springs from the chair in place of his replica and grabs the £100K jewel. His bedroom has a gramophone and secret passage to behind the curtain.
After the police take away the villains, Lord Cantlemere sweeps in. Unlike the Prime Minister and Home Secretary, he did not want Holmes. When tricked into insisting on arrest for whoever is found possessing the diamond, he finds the jewel in his pocket, and apologizes. Finally, Holmes can eat.
This adventure is notable for being the one of only two Arthur Conan Doyle Holmes stories, aside from a couple of humorous vignettes, to be written in third person. The other is "His Last Bow". "The Mazarin Stone" was written this way because it was adapted from a stage play, "The Crown Diamond", in which Watson hardly appeared. Its adaptation from the theatre also explains why the action in this story is confined to one room.
In the original play, the villain was Holmes's enemy Colonel Sebastian Moran of "The Adventure of the Empty House" infamy, not Count Negretto Sylvius.
I recommend this book to all readers who appreciate a well written mystery story. ( )