The Sign of the Four is set in 1888 and 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 and two corrupt prison guards It presents the detectives 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 Watsons future wife Mary Morstan According to Mary in December 1878 her f...