Arthur Conan Doyle's The Sign of the Four (1890) is the second Sherlock Holmes novel (following A Study in Scarlet, 1887). The opening chapter makes reference to the previous case, as Watson and Holmes sit in Holmes's study and Watson presents a brochure titled A Study in Scarlet, making an overt reference to the pair's previous case. Holmes, who is shooting cocaine, explains that he resorts to the drug only when his mind is not sufficiently stimulated. Sherlock showcases his flair for detective work when making observations about a watch held by Watson.
One Mary Morston, to whom her employer recommended Sherlock for his perspicacity in solving cases, comes to their apartment explaining that she has received a letter instructing her to go to the Lyceum theater at an appointed time. Morston also explains to Holmes and Watson that her father was an officer in India who died about ten years ago, shortly after returning to England in 1878. After this time, she received a pearl in the mail at the address where she was then working as a governess. The script of the address matches that of the more recent letter. The letter explains that Mary has been wronged, but that she shall have justice.
Watson and Holmes travel with Mary to the Lyceum, where they are taken to the house of a strange man named Thaddeus Solto. Solto explains that he is the son of Major John Solto, with whom Mary's father served in India. He and his brother have searched for a treasure belonging to their father, whose only evidence is a chaplet full of pearls, which, at Thaddeus's insistence, were sent individually Mary at month intervals in order that she never feel destitute. When their father died, a note reading, "The Sign of the Four" was attached to the body.
The group travels to the brother Bartholomew's home, but he has been poisoned. 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 have taken to 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 bribed him to allow their escape when they murdered an emissary who carries the so-called "Agra treasure" (from a tea plantation overseer fleeing from the Indian Rebellion of 1857). The plan ultimately fails, and the four men are arrested. Sholto goes to a penal colony where Majors Sholto and Morstan (Mary's father) are in stationed. Small bribes the two to let him escape to find the treasure in exchange for a promise of the proceeds. Sholto himself goes in search of the treasure, but absconds with it rather than returning for Morstan and Small, as promised.
Thaddeus's brother Bartholomew found the treasure after his father died before revealing its location. Meanwhile, Small tracked down Solto and his pygmy accomplice, Tonga, killed Solto (according to Small, this man was not instructed to do so, but rather it was a misunderstanding).
When Mary Morstan attempts to open the iron box, there is no treasure inside, and it is revealed that Small, haunted by memories of his original accomplices, cast it overboard.
Meanwhile, with the case thus solved, Watson has fallen in love with Mary, though Sherlock finds marriage too emotional and prefers his cocaine habit.
Miss Mary Morstan goes to Sherlock Holmes and Dr. Watson with something of a mystery. Her father, formerly an officer in an Indian regiment, sent her word from London that she was to meet him at a certain hotel. When she kept the appointment, her father failed to appear, and he has not been heard from in the ten years elapsed since that time. His only known friend in England was Major Sholto, a brother officer, but that gentleman disclaimed any knowledge of Morstan’s presence in London. For the past six years, Mary has received one large and valuable pearl on a certain date each year. That morning, she received a note asking...
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