The Sign of Four (1890) was the second novel featuring Sherlock Holmes written by Sir Arthur Conan Doyle. Doyle wrote four novels and 56 stories starring Holmes, whom many regard as the best-known literary character ever invented.
The Sign of Four has a complex plot involving service in colonial India, a stolen treasure and a secret pact among four ex-convicts. It presents the detective's drug habit and humanizes him in a way that had not been done before.
It has been filmed several times, both for motion pictures and television.