In 1888,[1] Dr. Watson remonstrates with Holmes about his cocaine usage. Holmes claims that his mind "rebels at stagnation", and that he needs a problem to solve in order to stimulate himself.
Miss Mary Morstan then arrives with a case. She explains that, in December 1878, her father, Captain Arthur Morstan, arrived in London, on leave from his post as a convict guard in the Andaman Islands. He requested her to meet him at the Langham Hotel, but was not there when she arrived. Mary contacted Major John Sholto, a retired convict guard who was her father's only friend in London; however, he denied having seen Morstan, and nothing further has been heard of the Captain. Four years later, Mary answered an anonymous newspaper advertisement that said it would be "to her advantage" that she reveal her whereabouts; after replying with her address, Mary received a valuable pearl in the post, a gift repeated once a year for six years. With the sixth pearl, she received a letter asking for a meeting, claiming that she is a "wronged woman".
Holmes takes the case, and soon discovers that Major Sholto had died in 1882; within a week of his death, Mary received the first pearl. The only further clue Mary can give Holmes is a map of a fortress found in her father's desk, appended with the words "The Sign of the Four: Jonathan Small, Mahomet Singh, Abdullah Khan, Dost Akbar," and four small cross-like symbols.
Following the letter's instructions, Holmes, Watson, and Mary go to the Lyceum Theatre; there, they meet a coachman who takes them to the house of Major Sholto's son Thaddeus, the anonymous sender of the pearls. He reveals that Captain Morstan did, in fact, visit Major Sholto, demanding his half of a treasure that Sholto had secretly brought back from India. In the ensuing quarrel, Captain Morstan suffered a heart attack and died, striking his head on the treasure box as he fell. Afraid he would be suspected of murder, Major Sholto buried the body and hid the treasure, leaving out a small gold chaplet studded with twelve pearls. Thaddeus and his twin brother Bartholomew knew nothing of these events, until a letter from India caused their father to faint, and sicken to his death. On his deathbed, he confessed the truth to them; he was about to reveal to them where the treasure was hidden, when a bearded man appeared at the window and the Major died of fear. The brothers tried and failed to catch the intruder; later on, they found a note pinned to the Major's body, which read "The Sign of Four". Thaddeus began sending Mary the pearls to make things right, and the brothers searched for the treasure. Six years later, Bartholomew found and withheld it; Thaddeus then contacted Mary so they could both confront Bartholomew and demand their shares.
The party, now accompanied by Thaddeus, heads to Bartholomew's house, Pondicherry Lodge, Upper Norwood. As they enter the house, the worried housekeeper reveals that Bartholomew has locked himself in his laboratory and refuses to come out. Mary Morstan stays downstairs to comfort the housekeeper, while the others rush up to the laboratory door; through the keyhole, they can see Bartholomew Sholto slumped in his chair, with a "fixed and unnatural grin" upon his face. Holmes and Watson break down the door, to discover Bartholomew in a state of rigor mortis. Upon further inspection of the body, Holmes discovers a poisonous thorn above Bartholomew's ear. The treasure box is also gone, though there is a hole in the ceiling where it used to be.
While the police wrongly take Thaddeus in as a suspect, Holmes deduces from footmarks and other clues that there are two persons involved in the murder: a one-legged white man named Jonathan Small, and a small Andamanese accomplice, who accidentally stepped in creosote. Borrowing Toby, a trained scent hound, from a naturalist, Holmes traces the pair to a boat landing. Learning that Small has hired a steam launch named the Aurora, Holmes, with the help of the Baker Street Irregulars and his own disguises, traces the boat to a repair yard. In a police launch, Holmes and Watson pursue the Aurora when it flees the yard; the islander attempts to shoot a dart at Holmes, and is shot dead himself. Small attempts to flee, running the Aurora aground, but is captured. However, the treasure box is now empty; Small, not wanting to surrender the gems, had scattered them into the Thames during the chase.
Small confesses that he was once a soldier of the Third Buffs in India, and lost his right leg to a crocodile while bathing in the Ganges. He then became an overseer on an indigo plantation; the 1857 rebellion occurred, and he was forced to flee to the Agra fortress. While standing guard one night, he was overpowered by Sikh troopers, who gave him a choice; be killed, or help them waylay Achmet, a disguised servant of an outlawed rajah, who had sent Achmet with a box of jewelry to the British for safekeeping. The robbery and murder took place, but the crime was discovered, although the hidden jewels were not. Small and his accomplices got penal servitude on the Andaman Islands.
Some years later, Small learned that Major Sholto and Captain Morstan, who were guards at the convict barracks, had lost money playing cards. Small saw his chance, and made a deal with the officers; Sholto would recover the treasure, and in return send a boat to pick up Morstan, Small, and the Sikhs so they could all meet and divide it. However, Sholto stole the treasure for himself, returning to England after inheriting a fortune from his uncle. Morstan went after Sholto but never returned, and Small vowed vengeance against Sholto. Four years later, Small escaped prison with the help of Tonga, an islander whose life he had once saved. It was the news of this escape that shocked Sholto into his fatal illness. Small arrived too late to hear of the treasure's location, but left the note in the room anyway as revenge for the treatment of himself and the Sikhs. When Bartholomew eventually found the treasure, Small only planned to steal it; however, a miscommunication led Tonga to kill Bartholomew before Small could stop him. Small decides the treasure brings nothing but bad luck to anyone who has it; to Achmet and Bartholomew, death; to Major Sholto, fear and guilt; and for Small himself, penal servitude for life.
Mary is left with no treasure, save the pearls; however, she and Watson have fallen in love over the course of the adventure, and the loss of the treasure has removed any barriers there might have been between them. Watson reveals that he has proposed to Mary and that she has accepted, much to the annoyance of Holmes.
