The Sign of the Four (1890) by Sir Arthur Conan Doyle is the second novel featuring Sherlock Holmes. Doyle wrote 4 novels and 56 stories starring the fictional detective. The story is set in 1888. The Sign of the Four has a complex plot involving service in East India Company, India, the Indian Rebellion of 1857, a stolen treasure, and a secret pact among four convicts ('the Four' of the title) and two corrupt prison guards. It presents the detective's drug habit and humanizes him in a way that had not been done in the preceding novel, A Study in Scarlet. It also introduces Doctor Watson's future wife, Mary Morstan.