Holmes and Watson are soon on the trail of a mysterious figure in black, whose astounding speed and agility make capture impossible. The same suspect is then implicated in another murder, when the servant of a visiting Russian grand duke is found terribly mutilated in a notorious slum. But what links the two crimes, and do they have anything to do with the suicide of an unpopular schoolteacher at a remote boarding school? So begins a case that will reveal the dark shadows that past misdeeds can cast, and test the companions to their limits...