The Sign of the Four (1890), also called The Sign of Four, is the second novel featuring Sherlock Holmes written by Sir Arthur Conan Doyle.
Set in 1888,The Sign of the Four has a complex plot involving service in India, the Indian Mutiny of 1857, a stolen treasure, and a secret pact among four convicts ("the Four" of the title) and two corrupt prison guards. It presents Holmes's drug habit and humanizes him in a way that had not been done in the preceding novel, A Study in Scarlet (1887). It also introduces Dr. Watson's future wife, Mary Morstan.
