The Sign of Four

The Sign of Four was published in 1890 and is the second novel to feature Sherlock Holmes and Dr Watson, following A Study in Scarlet. It is the book that introduced the elements readers now treat as furniture: Watson meets Mary Morstan and marries her by the end, Holmes is shown using cocaine when the cases are dull, and the partnership is established as a domestic arrangement as much as a professional one.

The plot is a colonial inheritance story. The Agra treasure was seized during the Indian Rebellion of 1857 and has been followed across continents by men who agreed to divide it and then broke their agreement, which means the mystery is as much about what the British were doing abroad as about who killed whom in a London suburb. Doyle uses the Andaman Islands and their inhabitants in a way that modern readers notice, and the book's attitude towards them is part of its period.

The novel is short and was written quickly, and it is often described as the most atmospheric of the four Holmes novels because of the chase down the Thames at the end. It has been adapted for film and television repeatedly, including a 1983 made-for-television film with Ian Richardson as Holmes. Readers coming to Holmes from the short stories usually find this the best of the long cases after The Hound of the Baskervilles, and it is the novel in which Watson meets the woman he marries.