'The Sign of the Four' is the second Sherlock Holmes novel. Its complex plot involves service in the 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 Sherlock Holmes' 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.