The tone of “The Red-Headed League” is primarily observational and contemplative. This is because the story is narrated by Holmes’s curious and even-keeled assistant Watson, who acts as a grounding force in the story (in juxtaposition with Holmes, who behaves in eccentric and unexpected ways). The following passage captures Watson’s contemplative tone about mid-way through the story, when it is clear that Holmes has put the pieces of the puzzling case together while Watson himself has not:
I trust that I am not more dense than my neighbours, but I was always oppressed with a sense of my own stupidity in my dealings with Sherlock Holmes. Here I had heard what he had heard, I had seen what he had seen, and yet from his words it was evident that he saw clearly not only what had happened but what was about to happen, while to me the whole business was still confused and grotesque. As I drove home to my house in Kensington I thought over it all.
Specific language contributes to the reflective tone in this passage, such as, “I was always oppressed with a sense of my own stupidity,” “To me the whole business was still confused and grotesque,” and “As I drove home […] I thought over it all.” In many ways, Watson is a stand-in for readers who similarly do not have Holmes’s gifts of logical and deductive thinking, and Watson's moments of curiosity and contemplation therefore encourage readers to pause and consider their own limitations. Rather than trying to discourage readers with this maneuver, Conan Doyle is hoping to inspire them to pay more attention and become more curious, as Watson does over the course of the Sherlock Holmes stories and novels.