Sherlock Holmes receives a cipher message from Fred Porlock, a pseudonymous double agent of Professor Moriarty. Holmes deciphers the message as a warning of a nefarious plot against a man surnamed Douglas, a country gentleman residing at Birlstone House. Some minutes later, Inspector MacDonald arrives at 221B Baker Street with news that Douglas was murdered the night before. MacDonald, Holmes, and Dr. Watson travel to Birlstone House to investigate.