Set in 1888,[1] The Sign of the Four has a complex plot involving service in India, the Indian Mutiny of 1857, a stolen treasure, and a secret pact among four convicts ("the Four" of the title) and two corrupt prison guards. It presents Holmes's drug habit and humanizes him in a way that had not been done in the preceding novel, A Study in Scarlet (1887). It also introduces Dr. Watson's future wife, Mary Morstan.

The drama begins when a woman arrives on Holmes's doorstep asking for his help finding an anonymous person who has been sending her mysterious gifts and letters. Holmes agrees to investigate-but soon he and Dr. Watson find themselves entangled in a deadly treasure hunt. Sherlock Holmes and Dr. Watson set out to solve a case brought to their attention by a young woman who has received a pearl each year on the same date. Now she has received a mysterious letter asking her to meet an “unknown friend”. She asks for Holmes and Watson to help her and the adventure begins. As the story evolves we come across mysterious notes with “the sign of the four”. Murder, treasure, and intrigue abound in The Sign of the Four. Holmes is his usual eccentric and insightful character in this book.

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