Although it's not readily apparent in this adaptation, in the original story, the Mormon religion plays a strong role in the character's motivations and Brigham Young plays a part in the flashback denouement. At the time Conan Doyle was heavily criticized by Mormons, who felt the story presented their religion in a bad light.
'A Study in Scarlet' was the first Sherlock Holmes novel.
Although the novel was published in 1887, this takes place in 1881.
The basic plot of this episode resembles that of the original novel, regarding the murders and Holmes's investigation. However, there is absolutely no reference to Holmes and Watson having met for the first time. The opening of the novel covers the introduction of the two characters after Watson has been de-mobbed from the army.