Last Updated September 5, 2023.
Arthur Conan Doyle's 1890 novel The Sign of Four is the second full-length Sherlock Holmes novel, written following the wild success of the 1887 A Study in Scarlet.
As Watson and Holmes sit in their study, their conversation turns to their previous case. The pair discuss Watson’s recently completed brochure on the subject, aptly titled A Study in Scarlet. Holmes, who is shooting cocaine, is distracted and absent-mindedly criticizes Watson’s efforts as sentimental and romantic, for deduction is about specificity and scientific accuracy and not intrigue and mystery, as Watson’s brochure implies. Embittered by his partner’s arrogance and impropriety, Watson criticizes Holmes’s drug use. The detective replies, confident that the drug is simply a solution for moments of under stimulation and does not affect his mental faculties, proving his claim by deducing several truths about a watch Watson places in front of him.
Their bickering is interrupted by one Mary Morston, to whom her employer recommended Holmes for his perspicacity in solving cases. She arrives at 221B, Holmes and Watson's Baker Street apartment, and explains that she has just received a letter instructing her to go to the Lyceum theater at an appointed time. The letter, she tells the pair, is not so strange and begins to narrate her complex personal history.
Morston explains to Holmes and Watson that her father, an officer in India, died ten years ago, shortly after his 1878 return to England. In 1882, just a handful of years after his death, she began to receive mysterious envelopes containing a single pearl from an unknown benefactor. For the last six years, this pattern has remained unchanged. That is, until now. The script of the most recent letter matches the previous letters', but its contents are new and ominous: Morston has been wronged but shall soon have justice. Despite the promise of answers, she hesitates to meet with her strange benefactor alone.
Watson and Holmes travel with Mary to the Lyceum, where they are taken to Thaddeus Sholto's home. Thaddeus explains that he is the son of Major John Sholto, with whom Mary's father served in India. Thaddeus and his brother, Bartholomew, have searched for a treasure belonging to their father. The only evidence of the treasure's existence is a chaplet full of pearls, which, at Thaddeus's insistence, were sent individually to Mary to ensure that she never feels destitute. He has sent the letter now because he believes Bartholomew has found the treasure and worries about the ramifications of its discovery. Thaddeus’s fears are not unfounded, for when their father died, a note reading "The Sign of the Four" was attached to the body, and a mysterious footprint appeared outside the window facing his deathbed. The group, now slightly more enlightened, travels to Bartholomew's home, only to discover his lifeless body, laid low by poison. Holmes uses dogs to track the scent from the crime scene and discovers that the men responsible for the crime have hired a boat and set off on the Thames.
Holmes enlists the help of the police to catch the barge, which carries one Johnathan Small and a pygmy man named Tonga. At the police station, Small explains that while stationed in India, three Indian men, imprisoned for their murder of an emissary who carried the so-called "Agra treasure," bribed him to allow their escape. The plan ultimately fails, and the four men are arrested. Sholto goes to a penal colony where Majors Sholto and Morstan, Mary’s father, were stationed. Small bribes the two men, and they promise to allow him to escape and search for the misplaced treasure. In...
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exchange, Small offers them a cut of the proceeds. However, Sholto betrays Small and Morstan, searching for the treasure himself and then absconding with his findings rather than returning as promised.
Guilt at his betrayal led him to an early grave, and Major Sholto attempted to reveal the location of the treasure and right his wrongs before his death, but ultimately failed to do so. Even though neither brother knew the treasure’s location, Bartholomew had successfully tracked it down. Small, who eventually escaped from the penal colony where he was kept prisoner, tracked down the Major and kept an eye on his sons, waiting for one of the Sholto men to reveal the treasure’s location. When Bartholomew succeeded in finding it, Small and his accomplice, Tonga, sought to steal the treasure that was rightfully theirs. Bartholomew’s death was an accident, and Small reveals that he regrets the time he has wasted and the pain he has caused in pursuit of the treasure.
Mary Morstan attempts to open the iron box said to contain the treasure, it is empty; Small reveals that he cast it into the Thames, for he felt haunted by memories of his original accomplices and believed the treasure was a curse. With the case thus solved, Watson falls in love with Mary and proposes they marry, though Holmes finds marriage too emotional and prefers his cocaine habit to companionship.