At the outset of the novel, Sherlock Holmes receives a mysterious cipher message from an agent of Professor Moriarty. Together Holmes and Watson decipher Moriarty's message which relates that a man named John Douglas, residing at Birlstone, is in danger. When they arrive at Brimstone, John Douglas is already dead, leaving Sherlock to use his detective skills to solve his biggest mystery yet.

Sherlock Holmes is famous for his intellectual prowess and is renowned for his skilful use of deductive reasoning, astute observation, and forensic skills to solve difficult cases. Deductive reasoning allows Holmes to impressively reveal a stranger's occupation. Similarly, by studying inanimate objects, he is able to make astonishingly detailed deductions about their owners. This mindset was a major innovation in the field of crime fiction, inspiring authors like Robert J. Sawyer, Neil Gaiman and Stephen King.