After being acquitted of murder, Sherlock Holmes' (Basil Rathbone) cunning nemesis Professor Moriarty (George Zucco) hatches a plot to steal the famous Star of Delhi from the Crown Jewels at the Tower of London. In order to distract the master sleuth from this crime of the century, Moriarty sends anonymous death threats to young socialite Ann Brandon (Ida Lupino in an early role), forcing Holmes and Dr. Watson (Nigel Bruce) to split up to protect the jewels and prevent her murder.