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 wronge...