Aboard a ship to Dover, James Goodram of the Royal Regent Museum, is lured away from his stateroom by the beautiful Naomi Drake so that she can steal the item he is transporting, the Borgia Pearl. Naomi conceals the stolen gem inside a camera, which she then convinces an elderly clergyman to carry through customs for her. Later, Naomi gives the camera to her boss, master criminal Giles Conover, only to discover that the pearl has been replaced by a note from the famous private detective, Sherlock Holmes, who was the clergyman in disguise. When the Borgia Pearl is later placed on display at the Royal Regent Museum, Holmes and his physician friend, Dr. John H. Watson, express concern about its safety, as Conover is still on the loose. Museum curator Francis Digby proudly demonstrates the museum's elaborate alarm system, but Holmes exposes a fatal flaw in the system when he simply turns off the electricity. Disguised as a workman, Conover overhears this and takes the opportunity to snatch the pearl while the alarm is off and escapes out a window. Conover is later captured, but as he no longer has the pearl in his possession, he is soon released by the police for lack of evidence. Soon thereafter, London is plagued by a series of murders, each victim of which is found with his back broken and surrounded by smashed china. Holmes quickly deduces that the murders are the work of the Hoxton Creeper, a member of Conover's gang thought killed during a prison escape from Devil's Island. After examining the broken bric-a-brac from the various murder scenes, Holmes and Watson discover that each victim owned a plaster bust of Napoleon. Retracing Conover's steps, Holmes and Watson learn that the criminal went into a plasterer's shop just prior to his capture. George Gelder, the plasterer, tells them that he was working on six Napoleons at that time, and Holmes quickly deduces that Conover placed the stolen pearl in one of the still-wet plaster busts. Upon learning that all the busts were sold to Amos Hodder, Holmes questions the art dealer, who tells the detective that his new assistant broke two of the busts, and that three others were purchased by the recent murder victims. Holmes quickly notices that Hodder's ledger has been altered and deduces that Naomi is his assistant in disguise. Holmes listens in on her phone conversations with Conover, thus acquiring the name of the fourth buyer, Dr. Julien Boncourt. Later, Conover and the Creeper break into the physician's home, where Holmes has disguised himself as Boncourt. After almost falling into the detective's trap, Conover gains the upper hand and orders the Creeper to search the house for the Napoleon bust. In a loud voice, Holmes states that Naomi has been arrested for murder and accuses Conover of allowing it to happen, which causes the lovestruck Creeper to turn on his boss and kill him. When the Creeper refuses to surrender to him, Holmes is then forced to kill the deformed giant. Police inspector Lestrade and his men then arrive on the scene, and Holmes breaks open the final bust and recovers the Borgia Pearl.