Which Sherlock Holmes story introduced Dr Watson as Holmes’s companion and narrator?

A Study in Scarlet introduced Dr Watson as Sherlock Holmes’s companion and narrator.

Arthur Conan Doyle’s first Holmes novel was published in 1887. It begins with Watson returning to London after serving as an army medical doctor in Afghanistan, where he meets the eccentric consulting detective Holmes through a mutual acquaintance. Their meeting leads to the famous Baker Street partnership.

The novel presents Holmes’s methods through Watson’s eyes, including observation, deduction and the examination of physical clues. Its title refers to the “scarlet thread of murder” that Holmes says runs through the case. The story also introduces Inspector Lestrade and Inspector Gregson of Scotland Yard.

Readers sometimes confuse this book with The Sign of the Four, which was Doyle’s second Holmes novel and further developed Watson’s relationship with Mary Morstan. A Study in Scarlet is also distinct from the later short-story collections that made Holmes internationally famous.