The Sign of the Four

One of the Sherlock Holmes novels written by Conan Doyle was The Sign of the Four, also called The Sign of Four. The Sign of the Four has a complex plot involving service in East India Company, the Indian Rebellion, 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. It also introduces Doctor Watson's future wife, Mary Morstan.