"The Sign of Four" is the second novel featuring Sir Arthur Conan Doyle's iconic detective, Sherlock Holmes, and his friend Dr. John Watson. Published in 1890, this mystery novel weaves a complex tale of love, betrayal, and colonial treasure. The story begins when Mary Morstan seeks Holmes's help in solving the mystery of her father's disappearance a decade earlier and a mysterious gift of pearls she has received each year since. The investigation reveals a twisted tale of a stolen treasure, a pact between convicts, and a secret pact symbolized by the "sign of four." The chase through London, a climactic boat chase, and Holmes's deductive powers make this an engaging read.