How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?
 Sherlock Holmes, Chap. 6, p. 111
 The Sign of the Four (1890), also called The Sign of Four, is the second novel featuring Sherlock Holmes by British writer Sir Arthur Conan Doyle. Doyle wrote four novels and 56 short stories featuring the fictional detective.
 In 1888, Dr. Watson remonstrates with Holmes about his cocaine usage. Holmes claims he is bored, that he cannot handle the day-to-day, that his mind "rebels at stagnation", and that needs a problem to solve in order to stimulate his mind; Miss Mary Morstan 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 former convict guard who was her father's only friend in London and was living in England; however, he denied having seen Morstan, who was never heard from again. Four years later, Mary answered an anonymous newspaper advertisement, asking for her whereabouts and saying that it will be "to her advantage" that she come forward, after replying with her address, she then 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. On the letter claiming that she is a "wronged woman" there is also an instruction for her to meet the mysterious writer at the pillar of the Lyceum Theatre and that she may bring a friend but no police. 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.
 Holmes, Watson, and Mary meet and go to the Lyceum Theatre where they meet a coachman who takes them through "unfashionable" regions of the city to the house of Major Sholto's son Thaddeus, the anonymous sender of the pearls. Apparently, Mary's father, Captain Morstan visited Major Sholto to demand 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 man fled, and the only evidence of his presence there was a singular footprint outside the window. Later on, when the brothers returned to the bedroom in which Major Sholto had died, the room had been searched; pinned to the Major's body was a note that 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 has contacted Mary so they can both confront Bartholomew and demand their shares.
 The party, now accompanied by Thaddeus Sholto, heads to Bartholomew's house, Pondicherry Lodge, Upper Norwood. When they arrive, the doorman McMurdo refuses to let the party in until Holmes reveals himself to McMurdo as an old boxing friend. As they enter the house, the housekeeper Mrs. Bernstone is sick with worry as she 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 on his chair sitting in an odd position with a "fixed and unnatural grin" upon his face. Holmes and Watson break down the door, to discover Bartholomew dead and in a state of "rigor mortis." Upon further inspection of the body, Holmes deduces that the body has been there for hours and discovers a poisonous thorn above Bartholomew's ear. While the police wrongly take Thaddeus in as a suspect, Holmes deduces that there are two persons involved in the murder: a one-legged white man named Jonathan Small, and a small Andamanese accomplice who 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; during the chase Small, not wanting to surrender the gems, had scattered them overboard into the Thames. 
 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.
 Twenty years later, Small learned that Major Sholto and Captain Morstan, who were guards at the convict barracks, had lost money playingcards. 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 a way of revenge for the treatment of himself and the Sikhs who together became known as the "Sign of Four." When Bartholomew 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. Because Mary no longer has the treasure and because it is lost, there will no longer be a class divide between the two meaning that they can become married. Watson reveals to Holmes that he has proposed to her and that she has accepted, much to the annoyance of Holmes, who professes that he will never marry.
 Sir Arthur Conan Doyle described how he was commissioned to write the story over a dinner with Joseph Marshall Stoddart, managing editor of the American publication Lippincott's Monthly Magazine, at the Langham Hotel in London on 30 August 1889. Stoddart wanted to produce an English version of Lippincott’s with a British editor and British contributors. The dinner was also attended by Oscar Wilde, who eventually contributed The Picture of Dorian Gray to the July 1890 issue.  Doyle discussed what he called this "golden evening" in his 1924 autobiography Memories and Adventures.
 The novel first appeared in the February 1890 edition of Lippincott's Monthly Magazine as The Sign of the Four; or The Problem of the Sholtos, appearing in both London and Philadelphia. The British edition of the magazine originally sold for a shilling, and the American for 25 cents. Surviving copies are now worth several thousand dollars.
 Over the following few months in the same year, the novel was then republished in several regional British journals. These re-serialisations gave the title as The Sign of Four. The novel was published in book form in October 1890 by Spencer Blackett, again using the title The Sign of Four. This edition included a frontispiece illustrated by Charles H. M. Kerr. The title of both the British and American editions of this first book edition omitted the second "the" of the original title.
 A German edition of the book published in 1902 was illustrated by Richard Gutschmidt. An edition published by George Newnes Ltd in 1903 was illustrated by F. H. Townsend.
 Different editions over the years have varied between the two forms of the title, with most editions favouring the four-word form. The actual text in the novel nearly always uses "the Sign of the Four" (the five-word form) to describe the symbol in the story, although the four-word form is used twice by Jonathan Small in his narrative at the end of the story.
 As with the first story, A Study in Scarlet, produced two years previously, The Sign of the Four was not particularly successful to start with. It was the short stories, published from 1891 onwards in Strand Magazine, that made household names of Sherlock Holmes and his creator.
 There have been multiple film and television adaptations of the book:
 A radio adaptation of the story was broadcast on New York radio station WGY on 9 November 1922. The cast included Edward H. Smith as Sherlock Holmes, F. H. Oliver as Dr. Watson, and Viola Karwowska as Mary Morstan. It was produced as part of a series of adaptations of plays, so it is likely that the script was based on an existing stage adaptation of the story (one was written by John Arthur Fraser in 1901 and another by Charles P. Rice in 1903).
 A six-part adaptation of the novel aired in the radio series The Adventures of Sherlock Holmes. Adapted by Edith Meiser, the episodes aired from 9 November 1932 to 14 December 1932, with Richard Gordon as Sherlock Holmes and Leigh Lovell as Dr. Watson.
 The book was adapted by Felix Felton for the BBC Light Programme in 1959. Richard Hurndall played Holmes and Bryan Coleman played Watson.
 In 1963, the story was dramatised by Michael Hardwick for the BBC Home Service as part of the 1952–1969 radio series, with Carleton Hobbs as Holmes and Norman Shelley as Watson.
 CBS Radio Mystery Theater aired a radio version of the story in 1977, starring Kevin McCarthy as Holmes and Court Benson as Watson.
 The Sign of the Four was adapted for radio by Bert Coules in 1989 as part of BBC Radio 4's complete Sherlock Holmes 1989–1998 radio series, with Clive Merrison as Holmes, Michael Williams as Watson, and featuring Brian Blessed as Jonathan Small.
 In 2016, the story was adapted as an episode of the American radio series The Classic Adventures of Sherlock Holmes, with John Patrick Lowrie as Holmes and Lawrence Albert as Watson.
 Paul Giovanni's 1978 play The Crucifer of Blood is based on the novel. The Broadway premiere featured Paxton Whitehead as Holmes and Timothy Landfield as Watson. The 1979 London production featured Keith Michell as Holmes and Denis Lill as Watson.
 The novel, like the rest of the Sherlock Holmes' novels, are set in Victorian Britain. The book features themes of "fear of the other", class divide, and colonisation; primarily focusing on the British Empire and its treatment of India.
 The Sign of Four became an English Literature exam text for GCSE's.