In the story, the Mazarin Stone, a yellow diamond which is part of the British Crown jewels, has been stolen. The brilliant consulting detective Sherlock Holmes has been hired by the British government to find and return the diamond. Holmes finds out that the gem has been stolen by Count Negretto Sylvius with the assistance of a boxer called Sam Merton. However, Holmes does not know where the diamond itself is being kept. The detective manages to fool the two men into thinking that they can talk freely about the stolen gem and manages to take it from them.