In the novel, Holmes and his loyal friend Dr. John Watson are approached by Miss Mary Morstan, who seeks their help in solving the mystery of her father’s disappearance ten years prior. Her father had been an officer of the Indian Army and had gone missing after returning to London from India. Miss Morstan has been receiving an anonymous gift of a pearl every year on the anniversary of her father’s disappearance, and she wants to find out the truth behind it.

As Holmes and Watson delve into the case, they discover a complex web of deception, murder, and treasure. They become entangled with a one-legged man named Jonathan Small, who reveals that he was part of a mutiny during the Indian Rebellion of 1857 and that a fabulous treasure was hidden as a result. The pearl gifts were meant to serve as a guide to finding this treasure.

Holmes and Watson follow Small’s clues and race against time to track down the hidden treasure and solve the mystery of Morstan’s disappearance. Along the way, they face danger, betrayal, and ultimately uncover the truth.

The Sign of Four is a thrilling and suspenseful novel that showcases Holmes’ deductive reasoning and expert detective skills. It explores themes of justice, revenge, and the consequences of greed. It is considered to be one of Conan Doyle’s best works and a classic example of the detective genre.