features match what Holmes knows about Clay's appearance. He is small, stout-built, and about thirty years old. 

Later when Holmes sees Wilson's assistant on the pretext of asking for directions to the Strand he is sure of his man. He tells Watson:

“Smart fellow, that....He is, in my judgment, the fourth smartest man in London, and for daring I am not sure that he has not a claim to be third. I have known something of him before.”

Obviously Clay would not be working at that obscure little pawnshop unless he was planning to commit a serious crime. Holmes sees the branch bank a short distance away and deduces that Clay is digging a tunnel and wanted to get Wilson out of the way with the Red-Headed League hoax. Holmes deduces that Clay is doing a lot of digging from the moist and wrinkled condition of the knees of his trousers. Later when Holmes, Watson, Mr. Merryweather the banker, and a policeman are waiting to surprise Clay in the bank's basement, Jones the policeman tells Merryweather about Clay's character and history.

“John Clay, the murderer, thief, smasher, and forger. He's a young man, Mr. Merryweather, but he is at the head of his profession, and I would rather have my bracelets on him than on any criminal in London. He's a remarkable man, is young John Clay. His grandfather was a royal duke, and he himself has been to Eton and Oxford. His brain is as cunning as his fingers, and though we meet signs of him at every turn, we never know where to find the man himself. He'll crack a crib in Scotland one week, and be raising money to build an orphanage in Cornwall the next. I've been on his track for years and have never set eyes on him yet.”

Clay is arrested in the act of attempting to steal 30,000 gold Napoleons with a confederate. He is certain to be hanged for murder, but in Victorian times he could have been hanged for his other felonies, including the attempt to steal the French gold. Clay is associated with the infamous Dr. Moriarty, who is Sherlock Holmes' arch-enemy. They will later engage in a death struggle at Reichenbach Falls in Switzerland, which is described in "The Final Problem."

Sherlock Holmes made a lot of money from some of his cases, and therefore he was in a position to lead an enviable life of luxury and to accept only clients whose personalities or problems interested him. Often he would work on what lawyers call a pro bono publico basis, meaning they were expending their time and effort "for the public good" and not getting paid. Holmes always seemed deeply concerned about the security of his country. He felt there was a perpetual war of good against evil and that it was his duty to do what he could to protect the innocent against the hordes of wrongdoers who threatened civilization.

When Jabez Wilson comes to him with his petty complaint, it is obvious that there is little money involved. Wilson only earned a total of thirty-two pounds, and he is obviously looking for a pro bono relationship with Sherlock Holmes, since he is unlikely to want to part with any of those thirty-two pounds. Holmes takes the case because it is unusual and also because he is already suspecting that there is something serious behind it. As is often the case, he draws on his wide experience with crime and criminals to speculate whether John Clay might be the assistant willing to work for half wages. In a later story the reader will learn that the notorious Dr. Moriarty was himself deeply involved in the attempt to steal the French gold and that John Clay was only working for him.

Other stories in which Holmes is working on a pro bono basis are the one about the Six Napoleons, the one about the Speckled Band, and the one about the Solitary Cyclist. He frequently complains that he needs constant challenges to his analytical powers to keep him from becoming bored and depressed. His reputation as a great detective and also a humanitarian often attracts humble clients who could hardly afford to pay the fees to which a consultant of his stature is entitled.

Early in that story, Holmes remarks to Watson "“You will remember that I remarked the other day, just before we went into the very simple problem presented by Miss Mary Sutherland, that "for strange effects and extraordinary combinations we must go to life itself, which is always far more daring than any effort of the imagination.”

When he's presented with the actual announcement that led Wilson into the adventure, we're told "Holmes chuckled and wriggled in his chair, as was his habit when in high spirits. “It is a little off the beaten track, isn't it?” "

This is evidence that Holmes loves the strange, the odd, the bizarre, and the apparently irrational; he takes pride in showing the rational and logical reasons behind the absurd.

"The Red-Headed League" is one of the most famous, popular, and unusual of the Sherlock Holmes stories. A pawnbroker named Jabez Wilson, who has shockingly red hair, approaches Holmes and tells him that he was admitted to an elite society that only accepts men with a certain shade of red hair. As his employment with the league, he is put in a bare room and asked to copy the Encyclopedia Britannica for a salary. When the league is abruptly dissolved, Wilson goes to Holmes, who eventually deduces that the copying work is a ploy to keep him out of his pawnbroker's shop while criminals dig a tunnel from it to the bank next door.

At the beginning of the story, when Watson first enters the room, Holmes hints at why the case interests him. He says,

I know, my dear Watson, that you share my love of all that is bizarre and outside the conventions and humdrum routine of everyday life.

He then goes on to say,

Now, Mr. Jabez Wilson here has been good enough to call upon me this morning, and to begin a narrative which promises to be one of the most singular I have listened to for some time.

It's obvious, then, that Holmes is drawn to the case because of its unique and bizarre aspects. This is made even more clear later, when the strange elements of the case cause Holmes and Watson to burst out into laughter and Wilson threatens to leave. Holmes says, "I really wouldn't miss your case for the world. It is most refreshingly unusual."

Holmes is interested in the case of the Red-headed League because it is funny, and because it is interesting.  He knows that someone is planning something, and he wants to find out what.

I really wouldn't miss your case for the world. It is most refreshingly unusual. But there is, if you will excuse my saying so, something just a little funny about it. (p. 10)

Holmes tells Watson that “the more bizarre a thing is the less mysterious it proves to be” (p. 10), and the case may turn out to be quite simple.  As soon as he visits the shop and realizes that the man’s assistant has been digging in the cellar into the bank next door, it turns out to be a simple case indeed.  He is able to stop the bank robbery.

Holmes suspects the the person filling the position of Jabez Wilson's assistant of being the master criminal.  Holmes is very much aware of the details important to this crime, especially the assistant's role in ensuring that Mr. Wilson is away from the house at specific days and times.  The assistant, who is called Spaulding (actually John Clay), acts rather obviously as a coordinator of events, which indicates the has knowledge of elements that require that Wilson be positioned in ways typically unusual to him.

When Holmes is visiting the neighborhood of Jabez Wilson's pawn shop with Watson, he rings the bell of the shop and John Clay opens the door to invite them in. Holmes merely asks for directions to the Strand. Toward the end of the story, when he is explaining his deductions to Watson over whiskeys and sodas back at Baker Street, he tells his old friend:

I hardly looked at his face. His knees were what I wished to see.

The worn, wrinkled and stained condition of the knees of Clay's trousers proved to Holmes that he was right in suspecting that Wilson had been gotten out of the way so that his assistant could be free for four hours a day to dig a tunnel. When Holmes walked around the corner and saw the branch of the City and Suburban Bank, he was sure the tunnel was being dug for the purpose of looting the bank. The most important event in the story was Holmes's asking for directions to the Strand and seeing the condition of John Clay's trousers. He was already sure that the shop assistant posing as Vincent Spaulding was the master criminal John Clay.