In an example of situational irony, the pawnbroker Wilson’s seemingly innocent assistant "Spaulding" turns out to be the criminal mastermind Clay. While Wilson believed that Clay helped him secure a job with the Red-Headed League because Clay cared about his boss’s financial well-being, it turns out that Clay made up the existence of the league to lure Wilson away from the office while he dug a tunnel toward a nearby bank in order to rob it from below.
The irony of this plot twist comes across in the following passage, in which Wilson recounts for Holmes how kindly his assistant reacted to his being offered a job with the League:
“‘Well, it is a little awkward, for I have a business already,’ said I.
“‘Oh, never mind about that, Mr. Wilson!’ said Vincent Spaulding. ‘I should be able to look after that for you.’
“‘What would be the hours?’ I asked.
“‘Ten to two.’
“Now a pawnbroker’s business is mostly done of an evening, Mr. Holmes, especially Thursday and Friday evening, which is just before pay-day; so it would suit me very well to earn a little in the mornings. Besides, I knew that my assistant was a good man, and that he would see to anything that turned up.”
As readers later learn, Clay’s seemingly kind response here is actually an act of manipulation, making Wilson’s claim that his assistant is “a good man” a painfully ironic statement. Wilson even goes as far as to tell Holmes, “There is no vice in him,” a truly ironic claim to make about a criminal mastermind. This is one of the many examples in the story of appearances not being exactly as they seem—while Clay appears young, innocent, and trustworthy, he is anything but.
