Part I: The Mystery of the Red-Headed League
* The introduction of Sherlock Holmes and Dr. Watson: The story begins with Watson, recently discharged from the army, searching for lodgings in London. He meets Sherlock Holmes, a brilliant but eccentric detective who has a reputation for solving seemingly impossible cases.
* The Red-Headed League: A red-haired man named Jabez Wilson visits Holmes with a strange story. He was recruited to a mysterious organization called the Red-Headed League, where he had to spend four hours a day copying passages from an old book. He was suddenly dismissed when the League inexplicably disbanded.
* Holmes investigates: Holmes, intrigued by the unusual case, investigates the Red-Headed League. He deduces that the League was a ruse, a way to keep Wilson away from his workplace while someone else robbed a bank.
* The discovery of the crime: The crime is revealed to be a robbery of a bank branch in the basement of Wilson's workplace.
* The arrest of the criminals: Holmes, using his deductive powers and knowledge of the crime, identifies the perpetrators: a pair of cunning criminals, John Clay and his partner, who had used the Red-Headed League to distract Wilson.
Part II: The Sign of Four
* The introduction of Mary Morstan: Watson introduces Holmes to Mary Morstan, a young woman who is seeking Holmes' help. She has received anonymous gifts of pearls for the past seven years and now has a mysterious letter inviting her to meet a mysterious person.
* The mystery of the missing treasure: Mary's father was a military officer who disappeared without a trace. She believes the pearls are connected to his disappearance and a lost treasure.
* The pursuit of the criminals: Holmes discovers that the pearls are connected to a stolen treasure from the Agra treasure trove. He, Watson, and Mary follow clues left by a letter and track the criminals to London's docks.
* The chase on the Thames: The criminals, led by Jonathan Small, attempt to escape by boat on the Thames. A thrilling chase ensues, with Watson and Holmes pursuing them.
* The revelation of the villain: Small and his accomplice, Tonga, are captured, but Small reveals a tragic backstory. His former comrade, Thaddeus Sholto, had stolen the treasure and kept it hidden.
* The final confrontation: The climax of the story takes place in Thaddeus Sholto's house. Small and Tonga attack the Sholto family, but the confrontation ends with Small's arrest and a tragic revelation about the missing treasure.
The resolution of the mystery: In the end, the truth about the stolen treasure, the missing father, and the mysterious letters is revealed. Mary is reunited with a long-lost brother and the case is closed. The story also introduces a significant theme: the lasting impact of past wrongs on present lives.