The stockbroker’s clerk in the title is young Hall Pycroft, and he is traveling to Birmingham by train with Holmes, whose services he has engaged, and Watson. During the journey, he tells them about the case. He was let go from his clerk’s job a while ago, and after a long and desperate search for a new job he finally landed a post at major London stockbrokers Mawson & Williams. However, shortly before Pycroft was due to take up the position, he was offered a better-paying job by a man named Arthur Pinner, who instructed him to travel to Birmingham to meet his brother Harry, the “promoter” of the Franco-Midland Hardware Company, at their offices. Flattered by the offer, Pycroft accepted the job, signing a declaration stating his willingness to join the company, and agreeing to not officially resign his post at Mawson & Williams.

In Birmingham, Harry took Pycroft to the company’s office, which turned out to be a couple of small, dusty, and sparsely furnished rooms. Pycroft began to work, but soon felt a vague sense of unease. He then observed that Harry had a gold filling on the same tooth as his brother, and suspected that they must be the same man. His suspicions aroused, Pycroft consulted Holmes, who agreed to probe his mysterious new employer.

When the trio arrive at Pycroft’s Birmingham office, with Holmes and Watson posing as job-seekers, they find Pinner reading a newspaper and looking highly distraught; he asks them to leave him alone for a moment. Hearing strange sounds, they burst back into the room and find him hanging from a hook on the door. After cutting him down and reviving him, Holmes deduces that Pinner has devised a fictitious post for Pycroft in Birmingham, to get him out of the way so a crime could be committed at Mawson & Williams in London.

In a dramatic scene, illustrated here in The Strand Magazine, Holmes, Watson, and Pycroft force their way into Pinner’s office and find him attempting to commit suicide.

The newspaper Pinner was reading reveals both the details of the crime and the reason for Pinner’s despair. His real brother, a notorious forger and safe-breaker called Beddington, had taken up Pycroft’s position at Mawson & Williams; it was easy to impersonate the young clerk, since no one in the company had actually met him. Then, the previous Saturday, Beddington had dallied in the office, murdered the guard, and tried to make off with a bag stuffed with almost £100,000 in bonds. But an astute policeman, surprised to see a man leaving the office so late with a bag, arrested him, and the crime was discovered. All that remains, the paper reports, is to catch Beddington’s brother, who usually works with him. “We may save the police some little trouble in that direction,” remarks Holmes.

Holmes seems surprisingly sympathetic toward Pinner, who, having been driven to suicide on hearing of his brother’s arrest, is now facing prison for his role in the crime. In fact, Holmes often stands outside the law, and is happy to see natural justice done instead. It is, of course, what puts him in Scotland Yard’s bad books, but it also saves him the trouble of having to build a legal case—once he has explained it, the reader is satisfied that justice has run its course. However, Holmes knows that in this case they must hand Pinner over to the police. The tale is also unusual in that, when he arrives at the solution, Holmes is far from the crime scene, and is only able to apprehend the accomplice. The felony itself took place within London, and it was only the sharp eyes of a policeman that led to the arrest of the perpetrator.