In Arthur Conan Doyle’s Sherlock Holmes novel The Sign of the Four, Holmes and Watson search for the Agra treasure.
The treasure is a large hoard of Indian jewels stolen during the Indian Rebellion of 1857. Jonathan Small becomes involved in its concealment after joining three Indian accomplices and a prison guard known as Tonga. Their agreement is called the “Sign of the Four.”
Mary Morstan contacts Holmes because her father disappeared years earlier and she has begun receiving valuable pearls anonymously. The pearls are connected to the hidden treasure and to the mysterious sender, Thaddeus Sholto.
The story was first published in 1890 under the title The Sign of the Six, but later editions used The Sign of the Four. The treasure should not be confused with the Baskerville inheritance in The Hound of the Baskervilles or the stolen crown jewels in other detective fiction.