The story is set in 1888. The Sign of the Four has a complex plot involving service in East India Company, India, the Indian Rebellion of 1857, a stolen treasure, and a secret pact among four convicts ("the Four" of the title) and two corrupt prison guards. It presents the detective's drug habit and humanizes him in a way that had not been done in the preceding novel, A Study in Scarlet (1887). It also introduces Doctor Watson's future wife, Mary Morstan. According to Mary, in December 1878, her father had telegraphed her upon his safe return from India and requested her to meet him at the Langham Hotel in London. When Mary arrived at the hotel, she was told her father had gone out the previous night and not returned. Despite all efforts, no trace has ever been found of him. Mary contacted her father's only friend who was in the same regiment and had since retired to England, one Major John Sholto, but he denied knowing her father had returned. The second puzzle is that she has received six pearls in the mail from an anonymous benefactor, one per year since 1882 after answering an anonymous newspaper query inquiring for her. With the last pearl she received a letter remarking that she has been a wronged woman and asking for a meeting. Holmes takes the case and soon discovers that Major Sholto had died in 1882 and that within a short span of time Mary began to receive the pearls, implying a connection. The only clue Mary can give Holmes is a map of a fortress found in her father's desk with the names of Jonathan Small, Mahomet Singh, Abdullah Khan and Dost Akbar. Ennuye par le manque d'enquetes, Sherlock Holmes s'adonne a la consommation de drogues pour s'occuper, ainsi qu'a diverses activites ennuyeuses, sous le regard de son colocataire, le docteur John Watson. En 1888, une jeune cliente blonde, Mary Morstan, debarque chez Holmes, et lui demande de l'aide. Elle lui explique qu'elle est gouvernante, et que c'est sa patronne qui lui a conseille de venir ici. Watson tombe sous le charme de la jeune femme qui expose son probleme: son pere, navigateur dans les Indes, est decede environ dix ans auparavant (disparu le 3 decembre 1878). Or, elle recoit, depuis six ans, et a cycle regulier, des perles de grande valeur. Et ce meme jour lui est parvenu une enveloppe ou un anonyme lui demande de venir a un rendez-vous, accompagnee si elle le souhaite, mais pas d'un policier. Holmes accepte de l'aider et, epaule de Watson, accompagne Mary audit rendez-vous. Ils decouvrent sur place que l'anonyme est le fils d'un ancien ami et collegue du capitaine Morstan. Celui-ci revele que leurs peres avaient decouvert un tresor de cinq cent mille livres, mais que ni le tresor ni les bijoux n'ont ete partages avec la fille de Morstan. L'homme explique enfin que son frere a decouvert le coffre hier, cache dans leur maison. Mais arrives sur les lieux, ils constatent la mort du frere et la disparition du coffre. Holmes et Watson commencent alors un dangereux periple qui les menera jusqu'a une course poursuite folle sur la Tamise..."