Sir Arthur Conan Doyle's The Sign of the Four is one of many fun mystery novels featuring the famous detective Sherlock Holmes and his sidekick, Doctor Watson. In the 1800s, Sherlock's newest client, Mary Morstan asks for Holmes' help because after returning from India, Mary's father had agreed to meet her at a hotel but never showed up. And he mysteriously went missing.

In addition, Mary has received a pearl every year since responding to an ad in a newspaper saying that she was wronged in some way. Sherlock discovers that Mary had read this advertisement shortly after her father's friend, Mr. Sholto, had died and that the two must be connected. The only other clue that Sherlock and Dr. Watson have to rely on is Mary's father's map of a fortress. The map has the names of four criminals on it; these criminals are the "four" alluded to in the title of the novel.

Wrapped in mystery, the tale of Mr. Morstan's disappearance is slowly revealed, as Sherlock discovers that Mr. Morstan and his friend, Mr. Sholto, had met to divide a treasure from India on the night of Mr. Morstan's disappearance. Mr. Sholto's son is the one who had been sending Mary the pearls, and the three must uncover who left the words "the sign of the four" in a note on Mr. Sholto's body.