The Sign of the Four (1890), also called the Sign of Four, is the second novel featuring Sherlock I lolmes written by Sir Arthur Conan Doyle. As a dense yellow fog swirls through the streets of London, a deep melancholy has descended on Sherlock Holmes, who sits in a cocaine induced harc at 221B Baker Street. His mood is only lifted by a visit from a beautiful but distressed young woman Mary Morstan, whose father vanished ten years before, Four years later she began to reveive an exquisite gift every year: a large, lustrous pearl. Now the has had an intriguing invitation to meet her unknown benefactor and urges Holmes and Watson to accompany her.