A Study in Scarlet is a detective mystery novel written by British author Sir Arthur Conan Doyle at the age of 27.

It was first published in 1887, and is the first story to feature the character of Sherlock Holmes, who would later become one of the most famous and iconic literary detective characters. The title of the book derives from a speech given by Holmes to his companion Doctor Watson on the nature of his work, in which he describes the story's murder investigation as his "study in scarlet": "There’s the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it."

When he wrote the story, Conan Doyle was a general practice doctor in Southsea, England, he had already published short stories in several magazines of the day, such as the periodical London Society. The story was originally titled A Tangled Skein, and was eventually published by Ward Lock & Co. in Beeton's Christmas Annual 1887, after many rejections. The author received £25 in return for the full rights (although Conan Doyle had pressed for a royalty instead). It was first published as a book in July 1888 by Ward, Lock & Co.

The story, and its main character, attracted little public interest when it first appeared. Only ten copies of Beeton's Christmas Annual 1887 are known to exist now and they have considerable value. Although Doyle wrote fifty-six 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 Four, published in 1890.

Plot summary

The novel is split into two quite separate halves. The first is titled Being a Reprint from the Reminiscences of John Watson, M.D., Late of the Army Medical Department. This part is told in first person by Holmes' friend Doctor John H. Watson and describes his introduction in 1881 to Sherlock Holmes through a mutual friend and the first mystery in which he followed Holmes' investigations. The mystery revolves around a corpse found at a derelict house in Brixton, England with the word "RACHE" scrawled in blood on the wall beside the body.

Holmes firmly resolves to solve the case despite the fact that he won't be given any credit for it. For this purpose, he makes up a plan using a wedding ring that had been lost at the crime scene. After placing an ad in the newspaper, asking for the ring owner, Holmes is visited by an old woman who claims the ring. Holmes follows "her", but she turns out to be a man in disguise, who then manages to escape.

The next day, Holmes is visited by one of the police detectives assigned to the case, who claims that the case has been solved and the murderer is now jailed. After the detective finishes explaining how he solved the case, a second police detective (Lestrade) arrives to announce that there has been a second murder - it is clear that the man the police have arrested is innocent. The police are now completely at a loss - both detectives have arrived at dead ends.

By way of reply, Holmes announces that he himself has solved the murder and will shortly arrest the killer. Pretending to be packing his bags for a journey, he asks the waiting cab driver to come and assist him with his luggage. As soon as the cab driver appears in his room, however, Holmes takes out his handcuffs and arrests the driver. Proudly he says, "Gentlemen... Let me introduce you to Mr. Jefferson Hope, the murderer of Enoch Drebber and of Joseph Stangerson.".

The second half of the story is called The Country of the Saints and jumps to the United States of America and the Mormon community, and incorporating a depiction of the Danites, including an appearance by Brigham Young in a somewhat villainous context. It is told in a third person narrative style, with an omniscient narrator, before returning in the last two chapters to Watson's account of Holmes' investigation, and then Holmes' own explanation of his solution. In these two chapters the relationship between the two halves of the novel becomes apparent. The motive for the crime is essentially one of lost love and revenge.