Reginald Musgrave is a bookkeeper at a large firm, and he has recently been blackmailed by a man named John Dorrington. Dorrington has threatened to expose Musgrave for embezzlement unless he pays him a large sum of money. Musgrave is innocent of the embezzlement, but he is afraid of Dorrington. He has heard rumors that Dorrington is a dangerous man, and he is worried that he will kill him if he doesn't pay the blackmail, and so comes to Sherlock Holmes for help.