Plot overview

A mystery with two connected stories

Arthur Conan Doyle’s The Sign of Four follows Sherlock Holmes and Dr Watson as they investigate Mary Morstan’s mysterious gifts and her father’s disappearance. What begins as a private family puzzle becomes a murder investigation and a pursuit of stolen treasure. Alongside the detective story runs a love story: Watson falls in love with Mary, but believes that her possible fortune prevents him from declaring his feelings.

Watson is both a participant and the first-person narrator. We usually discover events as he does, so Holmes’s reasoning can be clearer to Holmes than it is to either Watson or the reader. The novel gradually connects present-day crimes in London with much earlier events in India.

Chapter 1: Holmes before the case

At Baker Street, Holmes takes cocaine because he has no investigation to occupy his mind. Watson, a doctor and his friend, objects to the risks. Holmes discusses his detective methods and demonstrates them by examining a watch that belonged to Watson’s brother.

The chapter ends with Mary Morstan’s arrival. Conan Doyle introduces the detective and his relationship with the narrator before presenting the main mystery.

Chapter 2: Mary presents her mystery

Mary explains that her father, Captain Arthur Morstan, disappeared on 3 December 1878 after returning to London from service in India. His friend Major Sholto denied knowing that he had arrived. Since 4 May 1882, Mary has received a valuable pearl every year from an unknown sender. She now has six pearls and a letter inviting her to a meeting outside the Lyceum Theatre, where she may bring two friends but no police.

Holmes agrees to accompany her with Watson. Watson is immediately attracted to Mary, establishing the romantic plot alongside the investigation.

Chapter 3: A journey into uncertainty

Holmes discovers that Major Sholto died shortly before the first pearl arrived. He infers that Sholto’s heir may know about Morstan’s disappearance and be trying to compensate Mary. This is a reasoned explanation, not yet a fully established account.

Mary brings a plan found among her father’s papers. It bears the names Jonathan Small, Mahomet Singh, Abdullah Khan and Dost Akbar, together with the sign of the four. At the theatre, a driver meets the group and takes them through foggy London to an unfamiliar house. Watson loses his bearings, while Holmes follows their route. The journey delays the answers and builds suspense.

Chapter 4: Thaddeus explains the pearls

Their host is Thaddeus Sholto, one of Major Sholto’s twin sons. He explains that his father brought the Agra treasure to England. According to the Major’s deathbed confession, Captain Morstan came to claim his share, suffered a fatal heart attack during their quarrel and struck his head on the treasure-chest as he fell. The Major concealed the body because he feared being accused of murder and wanted to keep the treasure secret.

The Major asked his sons to give Mary her share, but died before revealing the hiding place when a frightening face appeared at his window. Afterwards, someone searched his room and left a message bearing the sign of the four. Thaddeus began sending Mary pearls, and now his brother Bartholomew has finally found the treasure in a concealed garret at Pondicherry Lodge.

The group sets off to claim Mary’s share. Watson fears that becoming a wealthy heiress would place her beyond his reach: his limited income and anxiety about appearing to seek her fortune make the treasure an obstacle to love.

Chapter 5: The case becomes a murder investigation

At Pondicherry Lodge, the housekeeper is frightened because Bartholomew has remained locked in his room all day. Holmes and Watson force the door and discover him dead, with a fixed, unnatural grin. Holmes finds a poisoned thorn above his ear. The treasure has disappeared, and a note bears the sign of the four.

This is the major turning point. Thaddeus’s explanation seemed to answer Mary’s original questions, but the discovery creates a new and more dangerous mystery: who killed Bartholomew, and how did the intruders enter and leave a locked room? Meanwhile, Watson and Mary seek comfort in each other, bringing the romantic plot closer to the detective action.

Chapter 6: Clues identify two intruders

Holmes examines the room rather than relying on appearances. A boot-print and the circular impression of a wooden leg indicate one intruder. Tiny bare footprints in the garret indicate another. He reconstructs their movements: the smaller accomplice entered through the roof and lowered a rope for the man with the wooden leg.

One intruder stepped in spilled creosote, a strong-smelling substance that a dog could follow. Holmes identifies the wooden-legged man as Jonathan Small, connecting the physical clues with the names on Mary’s plan and Major Sholto’s fear of men with wooden legs.

The police detective Athelney Jones instead arrests Thaddeus. Holmes promises to clear him and sends Watson to escort Mary home and fetch Toby, a dog with an exceptional sense of smell.

Chapter 7: Toby follows the wrong trail

Watson restrains his feelings for Mary because she is distressed and because he fears being mistaken for a fortune-seeker. He then collects Toby and returns to Holmes.

Holmes traces the accomplice’s route over the roof and finds a pouch of poisoned darts. Toby follows the creosote scent across London, but eventually leads the detectives to a barrel of creosote in a timber-yard. Their apparent progress ends in a comic disappointment. The episode shows that a genuine clue can still lead investigators in the wrong direction when another trail crosses it.

Chapter 8: The trail reaches the Thames

Holmes takes Toby back to the point where the trails crossed. This time the dog leads them to a riverside landing-stage. Mrs Smith’s information reveals that her husband, Mordecai Smith, left in his steam launch, the Aurora, after a visit from a wooden-legged man.

Holmes recruits the Baker Street Irregulars, a group of street boys led by Wiggins, to search both banks for the launch. He also concludes that Small’s accomplice comes from the Andaman Islands. His explanation uses a gazetteer containing racist descriptions: these reflect the prejudices within the novel’s Victorian world, not reliable facts about a people.

Chapter 9: A delay, then a breakthrough

The search fails to locate the Aurora, leaving Holmes restless and frustrated. Watson visits Mary, whose concern is for the wrongly accused Thaddeus rather than her possible wealth. Thaddeus is released after his alibi establishes that he could not have committed the crime.

Holmes searches the riverside in disguise. He returns dressed as an elderly sailor and briefly fools both Watson and Jones before revealing himself. Jones now accepts Holmes’s leadership, and they arrange a police launch for that evening. The interruption in the investigation makes the breakthrough more satisfying and prepares the reader for the chase.

Chapter 10: The chase down the Thames

Holmes explains how he found the Aurora hidden at Jacobson’s boatyard under the pretence of needing a minor repair. He reasons that Small wanted the launch concealed but ready for escape.

Holmes, Watson and Jones pursue it down the Thames. Small’s accomplice, Tonga, raises a blowpipe; Holmes and Watson shoot him, and he falls into the river. Small steers towards the bank and attempts to flee, but his wooden leg sinks into the mud. He is captured, and the iron treasure-chest is recovered.

The chase is the action climax: the long investigation becomes a direct confrontation with the fugitives.

Chapter 11: An empty chest brings a different reward

Small identifies Tonga as Bartholomew’s killer and says that the murder was not part of his plan. Watson takes the recovered chest to Mary, expecting to present her with a fortune. When they open it, however, it is empty.

Watson feels relief because the wealth he believed separated them has vanished. He declares his love, and Mary reciprocates. The detective pursuit has reached its outcome and the romantic plot is resolved, although the full history of the treasure remains to be told.

The ending of Chapter 11 changes the meaning of treasure. Watson narrates, ‘Whoever had lost a treasure, I knew that night that I had gained one.’ His figurative treasure is his relationship with Mary: emotional fulfilment replaces the jewels everyone has pursued.

Chapter 12: The earlier crimes explain the present

At Baker Street, Small reveals that he scattered the jewels into the Thames during the chase so that nobody else could possess them. He then tells his life story in an extended flashback.

Small lost his right leg to a crocodile while serving in India. During the rebellion of 1857, he took refuge at Agra and guarded a fort entrance with Mahomet Singh and Abdullah Khan. They threatened him into joining a plot involving their associate Dost Akbar. The four murdered Achmet, who was carrying a rajah’s jewels to safety, and hid the treasure. Small helped stop Achmet’s escape, making him an active participant in the killing. Their crime was discovered: the three Sikhs received penal servitude for life, while Small was condemned to death. His sentence was subsequently commuted to penal servitude for life as well.

In the Andaman Islands, Small offered Major Sholto and Captain Morstan a share of the treasure in exchange for helping all four prisoners escape. Sholto betrayed the bargain, taking the treasure to England and leaving the prisoners behind. Small later escaped with Tonga, whose life he had saved, and pursued Sholto for revenge. He was the face at the Major’s window. After years of watching the household, news that Bartholomew had found the treasure led to the theft and murder at Pondicherry Lodge. Holmes identifies the butler, Lal Rao, as Small’s inside accomplice, explaining how Small obtained information about the household.

Small presents himself as a wronged man, but his suffering does not erase his participation in murder. His account explains his motivation without making his claim to the stolen treasure morally rightful.

Finally, Watson tells Holmes of his engagement. Holmes objects to love as a threat to clear reasoning and reaches for his cocaine-bottle again.

Why the order of events matters

The novel tells the investigation forwards but reveals its causes backwards. Mary’s gifts lead to Thaddeus’s family history; Bartholomew’s murder leads to Small; Small’s final account explains the original theft and betrayal. By Chapter 11, Small has been caught, Tonga is dead and the chest has been opened. Chapter 12 then completes the explanation of why the London crimes occurred.

This delayed history links the London mystery to violence and the pursuit of wealth within Britain’s empire. What initially appears to be an unexplained family tragedy grows out of a much longer chain of greed, concealment and revenge.

The two main plots also end together: losing the jewels allows Watson and Mary to acknowledge their love. Watson’s life changes, whereas Holmes returns to the cocaine-bottle with which the novel began. This cyclical ending distinguishes the narrator’s personal fulfilment from the detective’s temporary satisfaction in solving a case.