Inspector Hopkins of Scotland Yard asks the brilliant consulting detective Sherlock Holmes to help him investigate the murder of Peter Carey, a former captain of a whaling ship whose nickname was Black Peter. The body of Peter Carey was found in a wooden hut in which he used to sleep, pinned to the wall by one of his own harpoons. A notebook with the initials J.H.N. was found at the scene of the crime. When a man named John Hopley Neligan is caught trying to break into Peter Carey's wooden hut, Inspector Hopkins believes that he has found the murderer. Holmes, however, is unconvinced.