A famous, cursed pearl is bought by the British Museum in London. But due to a rare mistake in judgement by Sherlock Holmes (Basil Rathbone), the treasure is swiped by a master thief (Miles Mander). Soon, Holmes learns of a series of brutal murders committed by a ghoulish maniac known as "The Creeper" (Rondo Hatton). Holmes must determine the link between the two criminals, the pearl and six plaster busts of Napoleon. Elementary!