In the second part of the story, Holmes hatches a plan to trick Irene Adler and get inside of her house. Although he remains loyal to Holmes and discreetly plays his part, Watson feels guilt over his involvement in the scheme. This reinforces the dramatic irony that marks the scene, as Watson feels bad precisely because he knows something the lovely, well-intentioned Irene Adler remains oblivious to. Watson's reflections on his guilt have the additional effect of cementing him as Holmes's foil:
I do not know whether he was seized with compunction at that moment for the part he was playing, but I know that I never felt more heartily ashamed of myself in my life than when I saw the beautiful creature against whom I was conspiring, or the grace and kindliness with which she waited upon the injured man.
Watson wonders whether it bothers Holmes to trick someone like Irene Adler. The detective's calculated manner, not to mention his proclivity for disguising himself to gather information about other people, makes it safe to assume that his conscience is not especially disturbed. Solving cases is an exciting game for Holmes, and he rarely expresses or displays compassion for others. Conversely, Watson is along for the ride but spends much more time ruminating on the consequences of his actions—moral and otherwise. The dramatic irony in this moment, and the contrasting ways their conniving affects the two men, raises the question of whether Holmes's detective talent is bolstered by his lack of empathy.
Ultimately, even if Watson feels bad, he does exactly what Holmes instructed him to do. He may feel some ambivalence over secrets and schemes, especially when it affects a seemingly innocent person like Irene Adler, but in this moment he concludes that he would rather engage in questionable moral activities than betray his friend.
And yet it would be the blackest treachery to Holmes to draw back now from the part which he had intrusted to me. I hardened my heart, and took the smoke-rocket from under my ulster.
The conflict in Watson's conscience reinforces the two men as foils for each other. Holmes appears willing to do anything for the purpose of solving a crime—even to the point of engaging in criminal activity himself. Watson, on the other hand, often reflects on how far it is appropriate to go. He is drawn to the rush of accompanying Holmes, but the plot against Irene Adler gives him pause. Their personalities and motivations complement each other: if Holmes is the duo's investigative mastermind, Watson is the the duo's conscience.
