The story is set in 1887 or 1888. The Sign of Four has a complex plot involving service in East India Company India, the Indian Rebellion of 1857, a stolen treasure and a secret pact among a guard and three convicts. It presents the detective's drug habit and humanizes him in a way that had not been done in the first novel, A Study in Scarlet. It also introduces Doctor Watson's future wife, Mary Morstan.