The Sign of the Four is the second Sherlock Holmes novel by Arthur Conan Doyle. The story begins when Mary Morstan seeks Holmes's help in solving the mystery of her missing father and the strange pearls she has been receiving anonymously. Holmes and Dr. Watson uncover a tale of betrayal, hidden treasure, and revenge, leading them on a thrilling chase through London. The novel deepens Holmes's character, explores Watson's romance with Mary, and showcases Holmes's brilliant deductive skills, making it a key work in the Sherlock Holmes canon.