The Sign of the Four (1890), also called The Sign of Four, is the second novel featuring Sherlock Holmes written by Sir Arthur Conan Doyle. Doyle wrote four novels and 56 stories starring the fictional detective.
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.
In 1888 a client, Mary Morstan, comes with two puzzles for Holmes. The first is the disappearance of her father Captain Arthur Morstan in December 1878 and the second is that she has received 6 pearls in the mail from an anonymous benefactor once a year since 1882, since she answered an anonymous newspaper query inquiring for her. With the last pearl she has received a letter remarking that she has been a wronged woman and asks for meeting. Holmes takes the case and soon discovers that Major Sholto — Morstan's only friend who had denied seeing Morstan — 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 with the names of Jonathan Small and three Sikhs, who are named Dost Akbar, Abdullah Khan, and Mahomet Singh.
Holmes, Watson, and Mary meet Thaddeus Sholto, the son of the late Major Sholto and Capt Morstan's Army friend who has sent her the pearls. Thaddeus remarks that his father had a paranoid fear of one-legged men and confirms that Mary's father had seen the Major the night he died. That night, in a quarrel about an Agra Treasure, Morstan — who was in weak health — suffered a heart attack. Not wanting to bring attention to the object of the quarrel to public notice, Sholto disposed of the body and hid the treasure. However his own health became worse when he received a letter from India. Dying, he called his two sons and confessed to Morstan's death and was about to divulge the location of the treasure when he suddenly cried "Keep him out!". The puzzled sons glimpsed a face in the window but the only trace was a single footstep in the dirt. On their father's body is a note reading "The Sign of Four". Both brothers quarreled over whether a legacy should be left to Mary Morstan and Thaddeus left his brother Bartholomew, taking a chaplet and sending its pearls to Mary. The reason he sent the letter is that Bartholomew has found the treasure and possibly Thaddeus and Mary might confront him for a division of it.
Sherlock Holmes took his bottle from the corner of the mantel-piece and his hypodermic syringe from its neat morocco case. With his long, white, nervous fingers he adjusted the delicate needle, and rolled back his left shirt-cuff. For some little time his eyes rested thoughtfully upon the sinewy forearm and wrist all dotted and scarred with innumerable puncture-marks. Finally he thrust the sharp point home, pressed down the tiny piston, and sank back into the velvet-lined arm-chair with a long sigh of satisfaction.
Three times a day for many months I had witnessed this performance, but custom had not reconciled my mind to it. On the contrary, from day to day I had become more irritable at the sight, and my conscience swelled nightly within me at the thought that I had lacked the courage to protest. Again and again I had registered a vow that I should deliver my soul upon the subject, but there was that in the cool, nonchalant air of my companion which made him the last man with whom one would care to take anything approaching to a liberty. His great powers, his masterly manner, and the experience which I had had of his many extraordinary qualities, all made me diffident and backward in crossing him.