Master criminal Giles Conover (Miles Mander) steals the famous "Borgia Pearl" from the Royal Regent Museum under the very nose of Sherlock Holmes and Dr. Watson, but when caught the pearl is not found on him, he is released.
Later, Holmes hears of an apparently motiveless murder. An elderly colonel is found with his back broken amid a pile of smashed china.
Holmes takes an immediate interest in the case as the unusual method of killing is that of "The Hoxton Creeper" (Rondo Hatton), known to be Conover's right-hand man.
Another murder occurs, of a little old lady, also surrounded by smashed china. Conover makes two attempts to kill Holmes, who surmises that Conover is desperately trying to recover the stolen pearl.
After a third killing Holmes finds the common feature of each: a bust of Napoleon. Conover, when pursued by the police, had fled through the workshop where they were being made, and hid the pearl inside one of six identical busts.
Holmes tracks down the vendor of the busts and finds out that one is still unaccounted for, as does Conover's accomplice Naomi. Conover and The Creeper arrive at the house of the owner of the final bust, only to find that Holmes has taken his place.
Overpowered, Holmes convinces The Creeper that Conover will double-cross him, and the Creeper turns on Conover and kills him, after which Holmes kills the Creeper, before the police finally arrive.
Holmes smashes the final bust and recovers the pearl "with the blood of five more victims on it".