The Sign of the Four

When an Englishwoman receives mysterious gifts of pearls and a letter promising to right wrongs done to her, she calls upon Sherlock Holmes and Dr. Watson to investigate. The Sign of the 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 four convicts (the Four of the title) 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.