Holmes has been tasked by the Prime Minister, the Home Secretary, and a certain Lord Cantlemere with retrieving a stolen Crown diamond worth £100,000, a figure one hundred times more than the Countess of Morcar’s stone in “The Adventure of the Blue Carbuncle”. Holmes knows who the thieves are but not where the stone is, so he has allowed them to remain at large despite the danger to himself. The ringleader, the half-Italian Count Negretto Sylvius, is a crack shot, so Holmes has placed a wax dummy of himself behind a curtain at the front window to foil any attempts to assassinate him. When the count turns up at the front door, Holmes sees a chance to resolve the case and sends Watson to summon Scotland Yard before receiving his visitor.

Sylvius is a foreign incarnation of big-game hunter Colonel Sebastian Moran from “The Empty House,” right down to the way Holmes lectures both men on the parallels between their form of hunting and his own. Both villains favor specially-engineered air rifles: Moran’s is made by “Von Herder, the blind German mechanic,” while Sylvius’s is the work of a similarly Germanic-sounding gunsmith, “old Straubenzee.” Admittedly, the count’s preferred quarry is Algerian lions rather than Indian tigers, but Moran’s mustache and large nose have found their way into Sylvius’s appearance. He also has a “cruel, thin-lipped mouth,” like the wicked Continental aristocrat, Baron Gruner, in the later story “The Adventure of the Illustrious Client”.

Holmes offers to let Sylvius and his accomplice, Sam Merton, go free if they surrender the stone, and duly withdraws, ostensibly to play the violin in his bedroom. Thinking they are alone, Sylvius tells Merton that he has the Mazarin stone on him—at which point the “dummy” dramatically springs to life, revolver in hand. Holmes has switched places with the wax figure via a secret door, and the violin music was courtesy of a gramophone. The thieves have been roundly outsmarted and are arrested.

In a familiar practical joke, Holmes then makes fun of the supercilious Lord Cantlemere by planting the diamond in his pocket while pretending to help him with his coat, and then mischievously accuses him of being its “receiver.” In an instant, Lord Cantlemere switches from saying that he has never believed in Holmes,to congratulating him on his incredible skills: “We are greatly your debtors, Mr. Holmes… I withdraw any reflection I have made upon your amazing professional powers.”

The story recycles plot details to the extent that it feels like a pastiche of “The Empty House” by someone other than Conan Doyle. It is dialogue-heavy, and much of the speech, lifted straight from the play, is hammy. The secret door is also a cliché. The story does, however, contain the occasional magnificent line, as when Holmes says, “I am a brain, Watson. The rest of me is a mere appendix.”

Count Sylvius, as illustrated by Alfred Gilbert in The Strand Magazine, prepares to attack the dummy but is instead greeted by Holmes’s “cool, sardonic” voice in the doorway.