London, 1890. 221B Baker St. A fine art dealer named Edmund Carstairs visits Sherlock Holmes and Dr John Watson to beg for their help. He is being menaced by a strange man in a flat cap - a wanted criminal who seems to have followed him all the way from America. In the days that follow, his home is robbed, his family is threatened. And then the first murder takes place.
At the end of November 1890, John Watson returns to Baker Street to stay with Holmes while his wife, Mary, is away attending to Richard, the ill son of her former employer. They receive a visit from Edmund Carstairs, a fine art dealer. Carstairs informs them that he is being followed by a mysterious man who has appeared to him three times and requested a meeting which he did not appear at.
Carstairs indicates that he believes the man is Keelan O'Donaghue, an Irish criminal who along with his twin brother Rourke ran a group called the Flat Cap Gang in south Boston. The gang had robbed a train carrying four paintings from his firm to a wealthy Bostonian named Cornelius Stillman, destroying them in the process and killing Carstairs' agent. Carstairs immediately left to Boston to meet Stillman, and convinced him to hire a Pinkerton detective named Bill McParland to track down the criminals. Rourke was killed in the ensuing gunfight, but Keelan escaped. Stillman was shot a week later, presumably by O'Donaghue, and Carstairs believes that he has now followed him to England to complete his vengeance for the death of his twin. Holmes listens to the case with interest, but tells Carstairs that he can do nothing more for him now; however, he says he thinks it unlikely that the man will try to harm him since he has not done so yet.
The following morning Holmes receives a telegram from Carstairs, which informs him that his home was burgled during the night and that he has informed the police - information Holmes was anticipating. He and Watson travel to Carstair's home, Ridgeway Hall, in Wimbledon. Carstairs is upset over Holmes' inaction, and introduces him to his wife, Catherine. She tells Holmes she heard a noise during the night and went downstairs, where she saw the burglar exiting through a window and raised the alarm. The only items stolen were a modest amount of cash and a necklace belonging to Carstairs' recently-deceased mother. Holmes notices from her accent that Catherine is American, and she informs him that they have not been married long - they met during Edmund's voyage back to England. Holmes is also introduced to Eliza, Edmund's spinster sister who occupies the top floor of the house. He then questions the servants, Mr and Mrs Kirby, who inform him that old Mrs Carstairs died of asphyxiation after leaving the gas running. He then investigates the garden, noting that due to the height of the window it would have been difficult to break into. As he and Watson are leaving, Eliza Carstairs tells them that she does not trust Catherine, who she suspects is after her Edmund's money, and that her mother's death was almost certainly a suicide caused by the marriage.
Holmes summons Wiggins and the Baker Street Irregulars to find the stolen necklace. On the suggestion of a boy named Ross, he instructs them to search the pawnbrokers near London Bridge Station. Meanwhile, he and Watson visit Carstairs' gallery and meet his partner, Tobias Finch. During their discussion of the stolen paintings Wiggins enters, announcing seen the man entering a cheap hotel in Bermondsey, and that he had posted Ross to watch him. Carstairs insists on accompanying Holmes and Watson, and together with Wiggins they take a carriage to the hotel. They find Ross outside, visibly frightened by their appearance, who tells them nobody has entered or left the hotel and then leaves hurriedly with Wiggins. On entering the hotel, the three men find the man dead by the door of his room, stabbed in the neck.
The next morning, Inspector Lestrade arrives at the hotel to investigate the murder. Holmes confirms that Carstairs identified the man as the one who had been following him. Holmes notes the absence of identifying documents apart from a cigarette case bearing the initials WM, though Watson points to a year-old bullet scar on the man's cheek as suggesting the man is in fact Keelan O'Donaghue. Holmes also notes they have no motive for the killing, but Lestrade argues that it may have the result of a argument with an accomplice. Observing the window, he realizes Ross would have had a clear view of the alley and may have seen the killer. Holmes tells Watson Ross' silence indicates he had a plan of his own, and is bothered by his fear on Holmes' arrival. He instructs Wiggins to find Ross, only to learn that he has disappeared. Wiggins tells Holmes that Ross had told him he recognized the man, and that he suspects he was intending to blackmail him; however, nobody had heard from him since. He also reveals that Ross was an orphan, and that he had been enrolled at a charitable school for boys before running away.
Holmes and Watson visit the school, Chorley Grange, and meet its headmaster, the Reverend Charles Fitzsimmons, and his wife Joanna. Reverend Fitzsimmons tells them that Ross had run away the previous summer, and had only been their pupil for a few months. He invites Holmes to tour the school and interview Ross' classmates. One boy reveals that Ross had a sister named Sally, who worked at a pub called the Bag of Nails in London. As they are leaving, Holmes notices an unusual building opposite and Reverend Fitzsimmons tells him it is used for performances.
References to other stories
- The story takes place seven weeks after "The Adventure of the Red-Headed League", and one week after "The Adventure of the Dying Detective".
- The opening deduction scene is a staple of several Holmes stories; however, Holmes' admiration of Edgar Allan Poe's character C. Auguste Dupin is contrary to a similar scene in A Study in Scarlet, where he considers Watson's comparison of him to the fictional detective an insult.
- Percy Trevelyan is Holmes' client in "The Adventure of the Resident Patient".
- Watson frequently references canon Holmes stories, sometimes by title. He explicitly names "The Adventure of the Blanched Soldier", and discusses "The Adventure of the Copper Beeches" with Moriarty. He also alludes to events from A Study in Scarlet, The Sign of the Four, "The Adventure of the Red-Headed League", "The Adventure of the Devil's Foot", "The Adventure of the Speckled Band", "The Adventure of the Final Problem", The Hound of the Baskervilles, "The Adventure of the Sussex Vampire, and the untold story of the "Giant Rat of Sumatra".
- Canon characters mentioned in the book include Godfrey Emsworth, Inspector Morton, Mrs Cecil Forrester, Stanley Hopkins, Jabez Wilson, Violet Hunter and Jephro Rucastle, Culverton Smith, and Jonas Oldacre.