''De Agra-Schat'' (The Sign of Four) is a detective novel by Arthur Conan Doyle, first published in 1887. The story follows Sherlock Holmes and Dr. John Watson as they investigate a case involving hidden treasures and the mysterious disappearance of Miss Mary Morstan's father. This novel explores themes of obsession, loyalty, and justice, while also providing insight into Holmes's character, including his struggles with cocaine addiction. It is notable for its intricate plot and the introduction of significant elements that would become staples in detective fiction.
