Chapters 2 to 4 of The Sign of Four focus on Miss Morstan's quest to uncover the mystery of her father's disappearance. She seeks Sherlock Holmes' help after receiving annual pearls from an unknown sender. Holmes and Watson discover connections to Major Sholto and the Agra treasure. They meet Thaddeus Sholto, who reveals his father's involvement with the treasure and Morstan's tragic fate. The chapters build suspense as clues about the treasure's location and the cryptic "Sign of Four" come to light.

Mary's Morstan gives Holmes some pearls and a letter. Holmes finds where the letter sender lives in London. Then they meet Thaddeus Sholto—he’s nervous. He says his dad, who was Mary’s dad’s old army friend, hid treasure. Now his brother Bartholomew has it. The group goes there, wondering about the treasure and Mary’s dad.

Miss Morstan tells Holmes and Watson about her father's disappearance, the strange occurrence of pearls she receives every year, and the anonymous letter she receives asking her to meet at the Lyceum Theatre.