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Scotland Yard Inspector Lestrade asks Sherlock Holmes for help in a seemingly simple case: to find the man who is smashing plaster busts of Napoleon. Lestrade suggests that they are dealing with a mad Napoleon-hater, but Holmes rejects this.

Finally, Holmes solves the mystery. They head to the owner of the last bust, where the detective buys it for £10. After they leave the bewildered man, Sherlock Holmes breaks the bust and finds a gem in the rubble, the black Borgia pearl.