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.
