The events of the story “Sign of Four” take place in London in 1888. During a forced idleness due to the lack of orders, the well-known detective consultant Sherlock Holmes, who lives on 221-6 on Baker Street, along with his friend Dr. Watson, sets out to him his deductive method, which he uses in solving crimes. Sherlock Holmes is convinced that an observant person is able to prove the existence of the Atlantic Ocean and Niagara Falls by logical conclusions, even if he had never seen them before and heard nothing about them. The smallest details and facts should be noted, since they are capable of making an invaluable contribution to the reconstruction of the full picture of events, the nature of the person and the circumstances of the crime.

Being a brilliant detective, Sherlock Holmes possesses encyclopedic knowledge in the field of criminal chronicle and chemistry, plays the violin well, has excellent fencing and boxing skills, is well versed in geology, anatomy, botany, has an extraordinary acting talent, but in the field of literature, philosophy, astronomy, his knowledge is zero. In the hours when he has nothing to do, he is gripped by deadly boredom and finds solace in morphine and cocaine.

Dr. Watson, a friend and companion of Sherlock Holmes, a former military doctor who served with the English army in the Indian war with Afghanistan and was wounded in this war, lives with Holmes in the same apartment and is a chronicler of all the cases his friend revealed.

To the great pleasure of Sherlock Holmes, his temporary idleness is interrupted by a certain Miss Morsten, a special twenty-seven year old with a spiritualized and kind face, testifying to the nobility and sympathy of the soul. She tells the detective about strange events that happened recently in her life, and asks him for help. In childhood, she lost her mother. The father, who served as an officer in India, sent his daughter to a boarding house in England. In 1878, that is, ten years ago, he arrived in England, as previously announced in a telegram. However, when Miss Morsten arrived at the hotel, she found out that her father had suddenly disappeared. He did not return the next day, never returned again. Then, starting in 1882, she suddenly began annually to receive from someone one very beautiful and large pearl. And on the day of her visit to Holmes, she received a letter in which she was asked to come to the Lyceum Theater in the evening, saying that she had been treated unfairly and that someone wanted to correct this injustice.

Sherlock Holmes and Dr. Watson go to the indicated place with her. Before leaving, she shows Holmes a strange note, found in the belongings of the missing Morsten, which depicted a plan of some room, with four crosses painted in a row in the same place, with crossbeams in contact with each other and with a pretentious signature: "sign of four." The person they met takes them in a cab to southern London. There they meet a small red-haired man with a shiny bald head. It turns out that he is one of the twin sons of Major Sholto, who died six years ago, Tadeusz Sholto. His father and father Miss Morsten once served together in India in the colonial forces. There, Major Sholto, mysteriously wealthy, resigned eleven years ago and returned to England with a rich collection of oriental rarities and a whole staff of native servants. The major did not reveal the secret of the acquisition of treasures and where they were kept until his death. Sensing her approach, he called his sons to him and told them how Captain Morsten died. It turned out that, having arrived in London ten years ago, he came to Sholto and they had a dispute over the division of treasures, about which Morsten knew and half of which was due to him. He, suffering from a sore heart, had an attack. Then he fell and, having hit his head on the corner of the treasure chest, he died. Fearing that he would be accused of murder, Sholto hid the captain's body and did not say anything to his daughter when, a few days later, looking for his missing father, she came to his house. Before his death, he also wanted to reveal to his sons where the casket itself was hidden, but a terrible face outside the window prevented him from doing this. He died, taking the secret with him to the grave. His sons, feeling a duty to Miss Morsten and wanting to rid her of at least need, began to send her each year one pearl of pearl beads, once removed by their father from the casket. For the time being, Tadeusz Sholto and his brother Bartholomew did not even know where the riches themselves were hidden. However, on the eve, after many years of unsuccessful searches, Bartholomew found them in the attic of his house, in a secret walled room. He reported this to Tadeusz. He, despite the objections of his brother, who inherited his stinginess from his father, decided to share the treasures with Miss Morsten. All four go to Bartholomew. However, they discover that he was killed by a poisonous thorn thrown into his neck, that the treasures were stolen and that a piece of paper with the “sign of four” was left at the crime scene.

Small details make Sherlock Holmes assume that two people are the criminals - a runaway convict named Jonathan Small, who has a wooden prosthesis instead of his right leg, and Number One, a savage from the Andaman Islands, is small, very angry and nimble. After he helped Small along with the casket to get down the rope from the window, he closed the shutters from the inside and got out through the attic. While running, he got his foot dirty in creosote, and Holmes, with the help of the sniffer dog Toby, followed his footsteps to the river. There he found out that the criminals boarded a hired Aurora boat. When Holmes’s plan to track down a boat with the help of a hired gang of boys breaks down, he, dressed as an old sailor, sets off in search of the Aurora and tries to find her on the docks. He succeeds. He calls to the aid of Scotland Yard Inspector Ethelny Jones investigating the murder, and they, along with Dr. Watson, chase a police boat and catch the criminals with a casket. When chasing a savage, he has to be killed, because he begins to shoot at his pursuers with his poisonous thorns. Dr. Watson takes the casket to Miss Morsten, but in the end it turns out that he is empty, which the doctor is extremely happy because the barrier that, in his opinion, has arisen between him and the young woman due to her alleged wealth, disappears. Now he can freely confess her love and offer his hand and heart. Miss Morsten finds his offer very attractive. Small, realizing that he would inevitably be caught, threw the jewelry into the Thames, because he did not want to let anyone else get it. The death of Bartholomew Sholto was not part of his plans, and he did not kill him, but the evil savage without Small's knowledge. To convince Sherlock Holmes and Ethelney Jones of this, he tells them the story of his life. In his youth, he enlisted as a soldier in the regiment, sent to India. However, he soon had to part with the service: when he bathed in Tanga, the crocodile bit off his leg above the knee, and he became a helpless cripple. Then, when he worked as an overseer on a plantation, a riot suddenly began in the country. Small hastened to Agra and joined the British unit hiding in the Agra fortress. He was entrusted with guarding one of the entrances to the fortress and was given two Sikhs. On the third night, the Sikhs captured Small and confronted him with a choice: to be with them or shut up forever. They told him about their plan: in the northern provinces there lived one very rich raja. He ordered part of his wealth to hide the servant Akhmet in the Agra fortress until the end of the war, so that in the event of the victory of the British to save at least this chest. The Sikhs and Akhmet’s companion, their accomplice, wanted to kill him and take possession of the casket. Small decided to join them and swore allegiance to them. All four carried out their plan. They hid the murdered Akhmet in one of the halls of the old fortress, where no one had ever entered. The chest was walled up in the wall of the same hall. Each of them received a note with a plan and symbolizing their fidelity to each other with the “sign of four”. However, subsequently they were all sentenced to life imprisonment for murder. While serving their sentences, they could not use their wealth. Then Small agreed with Sholto and Morsten, who were guarding the prison, that he would tell them where the casket was hidden, they would receive their share, and in return they would organize an escape for the four prisoners. Sholto, who went for the casket, deceived everyone and returned to England alone. Since then, Small began to live only with the thought of revenge. He escaped from prison with the help of a friend - a native named Tongo. In England, he made contact with one of Major Sholto's servants and waited for the right moment. It was Small who looked out the window to the dying major. Waiting in the wings, he stole the treasures. For the death of Bartholomew, he unfastened Tongo with a rope. That was the story of Jonathan Small.

Treasures did not go to anyone. Dr. Watson was given the wife of Miss Morsten, Ethelney Jones - fame for the solved crime, and Holmes was satisfied with an ampoule of cocaine.