"A Study in Scarlet" is an 1887 detective novel written by Arthur Conan Doyle. The story marks the first appearance of Sherlock Holmes and Dr. Watson, who would become the most famous detective duo in popular fiction.

When a post-war search for London lodgings introduces ailing doctor John H. Watson to quirky chemist Sherlock Holmes, their collaboration in the art of observation and deduction begins. Congenial flatmates and even better colleagues, the pair is quickly drawn into a case of murder and revenge that has Scotland Yard at their wits' end.

A gruesome murder is discovered with the word "RACHE" scrawled in blood, and Sherlock Holmes uses his unique powers of deduction to uncover a hidden story of passion, betrayal, vengeance and retribution.

Although Conan Doyle wrote 56 short stories featuring Holmes, "A Study in Scarlet" is one of only four full-length novels in the original canon. The novel was followed by The Sign of the Four, published in 1890. This was the first work of detective fiction to incorporate the magnifying glass as an investigative tool. Sherlock Holmes and Dr. Watson would go on to become one of the most well-known mystery-solving duos in literature.