“A Case of Identity,” published in September 1891, is the fifth episode in the series of four novels and 56 short stories in the Sherlock Holmes canon, written by Sir Arthur Conan Doyle. It follows the first two novel-length Holmes tales, A Study in Scarlet (1887) and The Sign of Four (1890), as well as the shorter stories “A Scandal in Bohemia” and “The Red-Headed League,” both of which appeared earlier that same year in The Strand magazine. This study guide cites the first volume of the 2003 Barnes & Noble edition of The Complete Sherlock Holmes.

The story begins inside Sherlock Holmes’s apartment on Baker Street, as Holmes converses with his longtime friend and (until recently) roommate, Dr. John Watson. It is the heyday of Holmes’s career as a consulting detective, and the two are discussing some of Holmes’s favorite subjects: criminal psychology and inductive analysis (though Holmes calls it deduction). As Holmes explains to Watson, the more complicated and interesting crimes are usually the smaller-scale, seemingly insignificant, everyday incidents that escape the public’s notice: “The bigger the crime,” Holmes says, “the more obvious, as a rule, is the motive” (226). Watson, not fully convinced, listens as his friend expounds upon the central importance of tending to unimportant or otherwise overlooked details for the key to a mystery since, as Holmes understands it, “there is nothing so unnatural as the commonplace” (225).

In the middle of their conversation, the two friends are visited by a fidgety young woman named Mary Sutherland, who approaches Holmes with a request for his help in a missing person case. Holmes quickly puts his intelligence on display by voicing observations about Miss Sutherland’s eyesight, occupation, and haste in coming to Baker Street; from observation alone, he knows she is a typist, and he knows she has shortsightedness. Miss Sutherland is perturbed by his uncanny insight, but he brushes it off, and the two launch into a lengthy conversation about the case at hand.

Miss Sutherland explains to Holmes that her stepfather, Mr. James Windibank, disapproved of her seeking out help, but as she is so desperate to find the missing person—Mr. Hosmer Angel—she came alone to Baker Street anyway. Holmes questions Miss Sutherland about her family and income, learning that Windibank, shortly after marrying Mary’s mother, insisted the family business be sold and took to managing both mother’s and daughter’s finances. To avoid being a burden to the family, Miss Sutherland works as a typist.

The conversation shifts to information about Hosmer Angel, whom Miss Sutherland met at a ball she attended against the wishes of her stepfather, who was supposed to be on a business trip to France. She and Angel formed a romantic relationship, and, even after Windibank’s return, they maintained a correspondence (handwritten on Miss Sutherland’s part, mailed to an obscure address, and typed documents from Angel). Angel, Miss Sutherland explains, was a soft-spoken, shy gentleman whose ophthalmic sensitivity inclined him to wearing sunglasses and coming out only at night.

Once Windibank had again left for France, Angel approached Miss Sutherland with a marriage proposal, which she accepted. However, on the day of the wedding and before the ceremony—and after a troubling conversation with Miss Sutherland in which he made her swear unconditional fidelity to their union—Angel vanished. Now, with no news from her fiancé and no means of finding him, the heartbroken Miss Sutherland concludes her account, able to provide only a few answers to Holmes’s questions. Before departing Baker Street, she leaves some items with Holmes: samples of Angel’s typed letters and the newspaper advertisement she wrote for the missing man.

Holmes and Watson, once more alone in the apartment, comment on the woman and her situation, and Holmes encourages Watson to attempt his own inductive observations. When Watson’s observations are inferior to Holmes’s, Holmes reveals how he discerned information from Miss Sutherland’s appearance: He knew she was a typist because her sleeves had a crease in them, caused by the edge of a table while she typed; and he knew she was nearsighted because she had small marks on her nose where eyeglasses must have pressed. He remarks that “all this is amusing, though rather elementary” (233). After inspecting the newspaper advertisement and Angel’s typed letters, Watson leaves Baker Street for the night, confident from their past experiences that Holmes will solve the mystery.

The following evening, Watson returns to find Holmes half asleep and distracted with a chemistry experiment. As Watson mentions the Sutherland case, another visitor appears at the apartment—this time James Windibank, Miss Sutherland’s stepfather.

Windibank, petitioned by Holmes to visit for a chat, apologizes to the consulting detective for his stepdaughter’s impulsiveness in asking for help in an impossible case. However, Holmes counters this by telling Windibank that in fact Angel will soon be found; and after a brief speech about the peculiarities of individual typewriter print, Holmes reaches for the door and locks the three of them inside, declaring he has caught the culprit.

Holmes then announces that Angel was in fact Windibank in disguise all along, an alter ego contrived to prevent Miss Sutherland from marrying and thus to keep her money in the family. Holmes points out the tell-tale clues: the similar irregularities in typed letter characters between Angel’s and Windibank’s letters, the two men’s matching descriptions (devoid of the disguise elements), the fact that neither was present at the same time as the other, and so on. Although there’s no legal recourse in such circumstances, Holmes expresses bitter disdain for Windibank and even reaches for a whip to lash out with, which sends the offender dashing from the apartment.

Holmes laughs and fills in for Watson the rest of the details. When Watson inquires after what will become of Miss Sutherland, Holmes declares it is best not to tell her—as he supposes she would never believe the truth anyway—and leave the situation to unravel on its own.