Charles Augustus Milverton makes his living by blackmailing wealthy people with incriminating letters acquired from disloyal servants and lovers. His latest victim is “the most beautiful debutante of last season,” Lady Eva Brackwell, who hires Sherlock Holmes to negotiate the return of some “imprudent,” “sprightly” letters, which threaten her engagement to an earl. As a debutante, Lady Eva would have been formally presented to the queen at a “coming-out ceremony,” before attending a full “season” of various social events with a view to securing a favorable marriage. The besmirching of her good name would mean the permanent ruin of her reputation, so the letters could have potentially dire consequences. When Milverton demands £7,000, Holmes offers him £2,000—but the blackmailer won’t budge. He wants to make “a severe example” of Lady Eva, leaving Holmes determined to retrieve the letters.

In an arguably ill-conceived plot device, Conan Doyle then has Holmes, usually a gentleman, behave with uncharacteristic caddishness by becoming falsely engaged to Milverton’s maid, Agatha, in order to learn the layout of the blackmailer’s house and details of his habits. “Surely you have gone too far?” cries Watson. Holmes replies: “You must play your cards as best you can when such a stake is on the table.” While he adds that he has a rival suitor who will no doubt step in the moment he is off the scene, clearly, to Holmes, the feelings and reputation of a maid count for less than those of a lady.

Holmes tells Watson that they must break into Milverton’s house and steal the compromising letters from his safe, which the two agree is a “morally justifiable, though technically criminal” act. Holmes has a sense of natural order, seeking to right wrongs, even if it means breaking the law. As a private detective he feels he can do this when he judges that the ends justify the means.

Charles Augustus Howell (1840–1890) as drawn by Frederick Sandys in 1882, eight years before his macabre murder.

Holmes and Watson break into Milverton’s house at night and witness a meeting between the blackmailer and a woman purporting to be the maid of a certain countess, offering to sell compromising letters. But very soon she reveals her true identity as the widow of one of Milverton’s victims, before committing the most violent act by a woman in the Holmes canon, pumping Milverton’s body full of bullets. Held back by Holmes, Watson realizes “that justice had overtaken a villain.” Holmes burns all of Milverton’s blackmail papers before they beat a hasty retreat.

The following morning, Inspector Lestrade visits Holmes to tell him that two burglars were seen fleeing the scene. Holmes mischievously suggests that the appearance of one of the felons “might be a description of Watson.” When he has had his fun, he declines to help Lestrade with the case, explaining, “my sympathies are with the criminals.” In a final twist, Holmes sees a photograph of Milverton’s killer in a shop window. But when Watson also recognizes her, Holmes puts a finger to his lips. Once again, his moral compass steers him away from adhering to the letter of the law.

The widow pours “bullet after bullet into Milverton’s shrinking body,” as Paget shows in The Strand Magazine.