A Study in Scarlet is divided into two parts. The setting of Part 1 is late Victorian London and it is narrated by Dr. John Watson. Part 2 takes place in the United States during the founding of Salt Late City in the 1840s by Mormons and is told from the perspective of the American Jefferson Hope.

A Study in Scarlet was published in 1887, at a time when British imperialism was at its height and London was the largest and most populous city in the world. The fact that much of A Study in Scarlet takes place in London during this time highlights the close link between detective fiction (which arose as a distinct genre in the 1840s) and urban settings. Detective fiction arose in the wake of the Industrial Revolution, after many people had moved from the countryside into the city in search of factory jobs. With more people than ever before living in close quarters to one another in cities, crime rates were on the rise. There was also an increased anxiety about crime, since one came into contact with strangers so much more often in cities than they did in the countryside. Whereas before people knew their neighbors, in the city, it was possible that their neighbor or the strangers they passed on the street were criminals. The increase in crime led to the creation of the first police forces in the early 19th century and, shortly thereafter, the rise of detective fiction. 

Many of the urban locations in this part of the novel are poverty-stricken and dilapidated. For example, in Part 1, Chapter 3, Watson and Holmes arrive at Number 3, Lauriston Gardens, a decaying, abandoned house where Enoch Drebber’s corpse has been found:

[The house] wore an ill-omened and minatory look. It was one of four that stood back some little way from the street […] two being empty. [They] looked out with three tiers of vacant melancholy windows, which were blank and dreary, save that here and there a “To Let” card had developed like a cataract upon bleared panes. A small garden sprinkled over with a scattered eruption of sickly plants separated each of these houses from the street.

Here, words like “ill-omened,” “vacant,” “melancholy,” “blank,” “dreary,” and “sickly,” create an ominous, suspenseful, and gloomy mood. The comparison of the “To Let” sign to a cataract suggests that this is a part of the city where crimes can happen outside the watchful eye of law enforcement, reflecting 19th-century anxieties about urban crime, particularly in poor neighborhoods.

Part 2, in contrast, takes place in Utah, away from any major cities, largely in the wilderness. Although many of the urban locations in Part 1 appear to be desolate and dangerous, the wilderness in Part 2 is depicted as actively hostile toward humanity and civilization. For example, in Part 2, Chapter 1, Doyle describes the Utah landscape in a threatening way:

In the central portion of the great North American Continent there lies an arid and repulsive desert, which for many a long year served as a barrier against the advance of civilization […] There are no inhabitants of this land of despair […] There are coyote skulls among the scrub, the buzzard flaps heavily through the air, and the […] grizzly bear lumbers through dark ravines.

Doyle’s choice to set this second part of the novel in the western United States reveals a 19th-century British view of the American West as a harsh and untamed wilderness full of danger. The landscape is described as being “a barrier against the advance of civilization,” suggesting that the Utah wilderness, and by extension the people living there, are less civilized than the English.