Several years after the mysterious disappearance of her father, Mary Morstan discovers an advertisement in a local paper requesting her own address. Her employer advises her to reveal it, and when she does she receives a valuable pearl by post. Presented with these facts and little else, Sherlock Holmes and Dr. Watson must unravel a mystery involving stolen treasure, political rebellion, India, and a pact made by four convicts, in Arthur Conan Doyle's second Sherlock Holmes novel.