The Sign of Four

The Sign of Four is the second novel featuring Sherlock Holmes written by A. Conan Doyle. The Sign of Four has a 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") and two corrupt prison guards. It presents the detective's drug habit and humanizes him in a way that had not been done in A Study in Scarlet. It also introduces Doctor Watson's future wife, Mary Morstan.