The Sign of Four

S4, E1: Every year since her father's disappearance and after replying to an advert in the paper for the address of 'Miss Mary Morstan', Miss Morstan has received a rare pearl. Summoned to meet her benefactor, she seeks the advice of Sherlock Holmes. With Watson in tow, they meet Thaddeus Sholto who tells of her father's death and the Agra treasure which he bought to England from India.