A jewel is missing from a museum, and Sherlock Holmes (Basil Rathbone) tries to find the thief as bodies pile up across the city. Holmes is sure that the crime is the work of Giles Conover (Miles Mander), and the recent string of murders suggests that the criminal, having stashed the pearl alongside the belongings of an unsuspecting Londonite, has ordered a henchman to find it. With Dr. Watson (Nigel Bruce) at his side, Holmes finds himself in mortal danger as he closes in on his man.