London, 1894. Sherlock Holmes is urgently called to the Grayson Gallery in Covent Garden to investigate an inexplicable and horrifying case: dozens of visitors have fallen dead in front of the painting titled The Eternal Man.

Very soon, Holmes and Watson are engaged in the hunt for a mysterious figure in black whose speed and agility make capturing her impossible.

This same enigmatic figure is believed to be involved in another crime: the servant of a Russian duke is found mutilated in a notorious slum in London. How can these two crimes be connected and what do they have to do with the suicide of an unpleasant teacher at a girls' boarding school?

This is the beginning of a case that hides dark and guilty secrets, a case that will put the limits and endurance of the two friends to a great test.