'The Sign of the Four' is an 1890 detective novel by Sir Arthur Conan Doyle and the second to feature Sherlock Holmes. The story begins in 1888 when Miss Mary Morstan seeks Holmes's help. She recounts the mysterious disappearance of her father, Captain Arthur Morstan, who vanished shortly after arriving in London. Despite inquiries to his friend Major Sholto, no information was found. Years later, Mary began receiving valuable pearls annually, accompanied by a letter requesting a meeting and identifying her as a 'wronged woman,' setting the stage for a complex mystery.