The Sign Of The Four

In 1888 a client comes with two puzzles for Holmes. The first is the disappearance of her father Captain Arthur Morstan in December 1878 and the second is that she has received 6 pearls in the mail from an anonymous benefactor once a year since 1882, since she answered an anonymous newspaper query inquiring for her. With the last pearl she has received a letter remarking that she has been a wronged woman and asks for meeting. Holmes takes the case and soon discovers that Major Sholto (Morstan's only friend who had denied seeing Morstan) had died in 1882 and that within a short span of time Mary Morstan began to receive the pearls, implying a connection. The only clue Mary can give Holmes is a map of a fortress with the names of Jonathan Small and three Sikhs,who are strangely named as Dost Akbar, Abdullah Khan, Mahomet Singh…
(Excerpt from Wikipedia)