Turkish baths were extremely popular among the well-to-do Victorians, and by the late 19th century, London was full of them. As Holmes and Watson enjoy a smoke in such an establishment on Northumberland Avenue (which runs between Trafalgar Square and the Embankment), Holmes takes out a note sent to him by a certain Colonel Sir James Damery, who has requested an audience with Holmes at Baker Street that day. Sir James’s name is recognized by Watson as “a household word in society,” and the Carlton Club in Pall Mall, where the note was written, was itself a well-known spot for high-ranking Conservative politicians and socialites in real life.

These high-society reference points suggest this case is likely to be one that will require the utmost discretion. Holmes himself notes that Damery has “a reputation for arranging delicate matters which are to be kept out of the papers”—that is, matters of social propriety. When Sir James arrives at 221B later that day, Watson describes his fastidiously well-turned-out figure, with top hat, frock coat, and varnished shoes. Sir James expresses his satisfaction at Watson’s presence—a nod to the chronicler’s own fame by this point.

Sir James then introduces his subject: the infamous Baron Albert Gruner, who he claims is the most dangerous man in Europe. Holmes immediately refers to Gruner as “the Austrian murderer”—clearly, both Holmes and Sir James are in agreement that (despite an official verdict to the contrary) the baron almost certainly murdered his ex-wife in Austria, and escaped prosecution only on technical grounds and due to “the suspicious death of a witness.”

Sir James then declares that he is working on behalf of a man who wishes to remain anonymous—the “illustrious client” of the story title. Holmes is perturbed by this desire for secrecy and presses Sir James to reveal the client’s identity, saying he is unable to commit to the case without knowing all the details. Damery remains resolute that he cannot disclose the client’s name, but assures Holmes that “his motives are, to the last degree, honourable and chivalrous” and that Holmes will be proud to serve him. He begs Holmes to listen to all the facts before making a decision.

The baron, who is irresistible to women and frequently takes advantage of them, has recently become engaged to the wealthy and attractive Miss Violet de Merville, the daughter of a famous British general. Given the baron’s violent and highly untrustworthy character, the match is sure to end in disaster for Violet. Holmes hears how deeply she has fallen for the handsome baron. “To say that she loves him hardly expresses it,” says Damery. He says she is “obsessed” by Gruner, who has managed to convince her of his innocence in any wrongdoing to which his name is attached. General de Merville is extremely concerned about the engagement, as is the anonymous client, who has known Violet since she was very young.

After hearing the full story, Holmes agrees to take on the case. He asks whether there is any further information he needs to know about the baron, and learns that he is artistic and an avid collector of Chinese pottery and has even written a book on the subject. Holmes observes that all great criminals have complex minds, citing the famous real-life Victorian villains Charles Peace—an inventor, violin virtuoso, and murderer—and “Wainwright,” that is, Thomas Griffiths Wainewright, a talented artist, critic, and poisoner.

Holmes’s first port of call is to contact his associate Shinwell Johnson—an ex-criminal who once served two jail terms but has since reformed. In creating this rather unlikely connection, Conan Doyle outlines the panorama of Victorian society in one story—with the “famous” high-society figure of Sir Damery at one end, and the shadowy figure of Johnson, with his contacts in London’s seedy criminal underworld, at the other.

Watson is not living at 221B at this point, but on Queen Anne Street. He meets the detective at Simpson’s-in-the-Strand—a favorite dining establishment of Conan Doyle’s in what was then one of London’s busiest thoroughfares. It is there that Holmes gives a description of the baron, whom he had brazenly called upon earlier. Holmes had demanded that the baron call off the marriage, but Gruner responded with threats, alluding to another former detective who had been horribly crippled after inquiring into the baron’s affairs. He also bragged that he has Violet de Merville in his thrall, and that, should Holmes call on her, she will not waver in her devotion to him.

Johnson tracks down a young working-class woman named Kitty Winter, who has suffered at the baron’s hands as one of his many former mistresses—although the reader never finds out precisely how. She tells Holmes of a “beastly book” in which the baron records the truth of all his misdeeds. He keeps it in his inner study, secreted behind another room containing his “Chinese crockery,” as she refers to his porcelain collection. Her own down-at-heel status, so far from that of Violet de Merville, is further evidence of the baron’s indifferent womanizing. “This man collects women,” Kitty tells Holmes, “and takes a pride in his collection, as some men collect moths or butterflies.” She recalls two other murders that the baron laid claim to while she was his lover, and also describes to Holmes in detail the location of the baron’s “inner study,” where she believes his secret book is kept.

The next evening, at Simpson’s again, Holmes tells Watson how he and Kitty visited the de Merville residence in upmarket Berkeley Square, Mayfair, which – in a show of his typical antipathy for houses of the wealthy—Holmes describes as “one of those awful gray London castles which would make a church seem frivolous.” Holmes says that when he tried to dissuade Violet from marrying the baron, she accused him of being a mercenary, a paid agent: as far as she is concerned, he is the immoral one. Even the plain-speaking Kitty could not convince her, and the visit was a failure.

The gentlemen’s dining room at Simpson’s-in-the-Strand, where Holmes and Watson dined. Women were forbidden to use this paneled street-level dining room until 1986.

Two days later, Watson is walking along the Strand when he sees a newspaper billboard announcing: “Murderous attack upon Sherlock Holmes.” Both the experience, and Watson’s stunned response, recall a moment in Conan Doyle’s own life. In his 1924 autobiography, Memories and Adventures, the author reports how he learned of the death of his friend Robert Louis Stevenson: “I cannot forget the shock that it was to me when driving down the Strand in a hansom cab in 1896 I saw upon a yellow evening poster ‘Death of Stevenson’. Some-thing seemed to have passed out of my world.”

Holmes is not in fact dead, but he has been violently assaulted in an attack that took place that day on Regent Street. His assailants escaped through the Café Royal—as popular a haunt with writers as Simpson’s, and, like Simpson’s, still there to this day—into the grimy alleys of Soho, which at that time was still a poor, murky, and very overcrowded area. There is another Stevenson link here. In Stevenson’s story Strange Case of Dr. Jekyll and Mr. Hyde (1886), the whole mystery turns on Dr. Jekyll’s grand residence having an incongruously “sordid” back door, leading onto a shadowy neighborhood—this is the door that his evil alter ego, Mr. Hyde, uses. Stevenson is thought to have based Dr. Jekyll’s house on that of the eminent 18th-century Scottish scientist and surgeon John Hunter (1728–1793), whose own residence in Leicester Square was barely a stone’s throw from the Café Royal, and would have backed onto the same seamy alleyways.

Holmes has a nasty head wound but will survive, thanks to the care of famous surgeon Leslie Oakshott. However, he asks Watson to greatly exaggerate his condition to all the press, and tell them he is dying so that anyone reading the papers will think he is off the case. However, when reports appear that the baron is soon to be traveling to America prior to the wedding, Holmes’s hand is forced, since he knows that the baron will take the incriminating book with him. With typical indifference to Watson’s professional duties, Holmes tasks him with learning everything he can about Chinese pottery in just 24 hours. He then gives him a priceless Ming saucer (provided by the mysterious client), with the request that he try to sell it to the baron. At the time Holmes does not explain why, but it later becomes clear that he needs Watson to distract the baron so he can break into the inner study and find the book. Watson duly begins to memorize as much information on Chinese pottery as he is able.

Throughout the story, Holmes and Watson visit real-life locations, most of which are concentrated in central London. Baker Street, where Holmes lives, is slightly north of the area shown on the map.

Tension increases at the baron’s luxurious residence as the highly suspicious host tests Watson, who is posing as Dr. Hill Barton, the ceramics expert, with excessively tricky questions on the Emperor Shomu and the Northern Wei dynasty. Watson keeps his cool, but the baron quickly sees through his bluff, rightly guessing that Holmes has sent him, and is enraged. Just as he is about to attack Watson in fury, the baron is alerted to a noise from his inner study. Dashing inside, he finds Holmes, who escapes through a window. Baron Gruner chases him into the garden, but as he follows him Watson sees a woman’s arm fly out from a bush, and, Gruner utters “a horrible cry” and falls, clutching his face. Watson rushes to his aid but finds that his face is being eaten away by acid. The hand, as it turns out, was Kitty Winter’s: Holmes had taken her with him to help locate the inner study but, acting of her own volition, she had seized a chance to take revenge on her past lover, and threw a measure of vitriol in his face, making a mangled mess of the baron’s once handsome features.

Back at 221B, it is clear that Holmes and Watson have moral qualms over the violence dished out to the baron. Although he is a multiple murderer, the acid attack is perhaps disproportionately vicious. “The wages of sin, Watson—the wages of sin!” intones the clearly shaken Holmes, in an echo of the Book of Romans in the New Testament.

Thanks to Watson’s keeping the baron talking for just long enough, Holmes is now in possession of the baron’s “lust diary,” which he stole from the inner study. This, Holmes believes, will finally open Violet’s eyes and put a stop to the marriage. He is of the clear opinion that the baron’s disfigurement alone would likely have the opposite effect as “she would love him the more as a disfigured martyr.”

In Holmes’s view, the state of being “madly in love” is equivalent to madness pure and simple, and indeed this story is peppered with references to women’s irrationality. Early on, Kitty Winter declared her willingness to risk death in order to take revenge, saying it with an “intensity of hatred,” Watson notes, “such as woman seldom and man never can attain.” And Kitty’s final, fateful deed plays out that reckless impulsiveness magnificently.

Three days later, the marriage between Violet and Baron Gruner has been called off. Meanwhile, a newspaper reports that Kitty will be brought before the courts for her crime. There is also a nod toward a degree of benign corruption here: although Holmes risks prosecution for burglary, Watson feels sure that the eminence of their client will make the law “elastic.”

Even at the end of the story, the reader never learns the identity of the client. When Watson realizes who it is, after seeing the “armorial bearings” on his carriage, Holmes silences him: “It is a loyal friend and a chivalrous gentleman. Let that now and forever be enough for us.”