In "The Sign of Four," Mary Morstan encourages Dr. Watson to get involved in her case due to her trust in him and his abilities. She approaches Watson after receiving mysterious pearls and learning about her father's disappearance. Realizing that she cannot solve the case on her own, she turns to Watson for help, knowing that he is capable and reliable. This action signifies her reliance on Watson’s intellect and character. Throughout the story, Mary’s faith in Watson allows him to be more than just an observer—he plays an active role in solving the mystery.