The Sign of Four, the second novel featuring Sherlock Holmes, was written by Sir Arthur Conan Doyle and first published in Lippincott's Monthly Magazine in February 1890. Set in 1888, the novel presents a flashback of characters working for the East India Company and the Indian Rebellion of 1857, involving a treasure heist, four convicts, and corrupt prison guards.

It is in this narrative that Doyle first delves into Holmes's drug addiction, providing a more human aspect to his character than seen in the preceding novel, A Study in Scarlet. This story also introduces Dr. Watson's future wife, Mary Morstan, who brings a perplexing mystery to Holmes—a mystery that he eagerly embraces as a stimulating challenge.

Mary Morstan seeks Holmes's help to solve the enigma surrounding her father, Captain Arthur Morstan, who disappeared under mysterious circumstances ten years prior upon returning to London from India, where he and his friend Thaddeus Sholto had discovered a vast treasure.