The Sign of the Four hides a secret pact among four convicts and two corrupt prison guards. According to Mary Morstan, her father requested her to meet him at the Langham Hotel in London upon his safe return from India. When Mary arrived at the hotel, she was told her father had gone out the previous night and not returned. Despite all efforts, no trace has ever been found of him. The second puzzle is that she has received six pearls in the mail from an anonymous benefactor, one per year since 1882. With the last pearl she received a letter remarking that she has been wronged and asking for a meeting. Holmes takes the case and soon discovers that Major Sholto, a friend of Mary's father, had died in 1882 and that within a short span of time Mary began to receive the pearls, implying a connection. The only clue Mary can give Holmes is a map of a fortress found in her father's desk with four names.

Arthur Conan Doyle (1859-1930) was a British writer best known for his detective fiction featuring Sherlock Holmes. The Sherlock Holmes stories are generally considered milestones in the field of crime fiction. Doyle is also known for writing the fictional adventures of Professor Challenger and for propagating the mystery of the Mary Celeste. He was a prolific writer whose other works include fantasy and science fiction stories, plays, romances, poetry, non-fiction and historical novels.