The Sign of the Four

The Sign of the Four is the second Sherlock Holmes book. Its complex plot involves 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 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.