The story revolves around the case of Miss Mary Sutherland, a woman with a substantial income from the interest on a fund set up for her. She is engaged to a quiet Londoner who has recently disappeared. Sherlock Holmes's detective powers are barely challenged as this turns out to be quite an elementary case for him, much as it puzzles Watson.
The fiancé, Mr. Hosmer Angel, is a peculiar character, rather quiet, and rather secretive about his life. Miss Sutherland only knows that he works in an office in Leadenhall Street, but nothing more specific than that. All his letters to her are typewritten, even the signature, and he insists that she write back to him through the local Post Office.
The climax of the sad liaison comes when Mr. Angel abandons Miss Sutherland at the altar on their wedding day.
Holmes, noting all these things, Hosmer Angel's description, and the fact that he only seems to meet with Miss Sutherland while her disapproving youngish stepfather, James Windibank, is out of the country on business, reaches a conclusion quite quickly. A typewritten letter confirms his belief beyond doubt. Only one person could have gained by this: Mr. James Windibank. Holmes deduces "Angel" had "disappeared" by simply going out the other side of a four-wheeler cab.
After solving the mystery, Holmes chooses not to tell his client the solution, since "If I tell her she will not believe me. You may remember the old Persian saying, 'There is danger for him who took the tiger cub, and danger also for who snatched a delusion from a woman.' There is as much sense in Hafiz as in Horace, and as much knowledge of the world." In this, however, he can be accused of not fulfilling his professional duty for which he was paid – namely, to investigate the matter to which she set him, provide her with the results and let her decide what to do with them. Holmes does advise his client to forget "Mr. Angel"; Miss Sutherland refuses to take Holmes' advice and vows to remain faithful to "Angel" until he reappears – for at least ten years.
Holmes predicts Windibank will continue a career in crime and end up on the gallows.
This story was the basis for the third Holmes adventure (filmed in 1921) in the silent film series starring Eille Norwood.
In 2001, this was the basis for the ninth episode of the second season of the animated television series Sherlock Holmes in the 22nd Century.
This story was adapted for the radio at least three times: starring Tom Conway and Nigel Bruce in 1948; starring John Gielgud and Ralph Richardson in 1954; and in 1990 as part of Bert Coules' complete radio adaptation of the canon, starring Clive Merrison as Holmes and Michael Williams as Watson, and featuring Susannah Corbett as Mary Sutherland.
In 2014, it is seen in "The Empty Hearse", from the BBC television series Sherlock, as one of the cases Sherlock works on with Molly Hooper assisting him. Mary Sutherland portrayed by Elizabeth Coyle. Molly Hooper portrayed by Louise Brealey.
In the fourth episode of the 2014 Japanese puppetry series Sherlock Holmes, Mary Sutherland is a female pupil of Beeton School. She is in love with the senior Hosmer Angel who suddenly disappears in a cave at the back of the school. Holmes, a pupil who lives in room 221B of Baker Dormitory, suspects that Angel and Windibank, one of the childhood friends of Sutherland, are the same person and he and Watson find out that there is no pupil called Hosmer Angel in the school. Holmes appreciates Watson for consoling the broken-hearted Sutherland. Watson tells Holmes, who criticises novels as in the original story, that he is wrong to do so because various things can be learned from novels including how to understand the female mind.
The story was adapted as the beginning of the third case in the 2016 Frogwares videogame Sherlock Holmes: The Devil's Daughter. Watson is not present when Mary Sutherland arrives and Sherlock is accompanied by Orson Wilde, an American actor training to play him. The player can come up with different conclusions to the case and also make a choice concening her the results of his deduction.
Colin Dexter, known for writing the Inspector Morse novels, wrote a short story based on this called "A Case of Mis-Identity", in which Holmes's brother Mycroft is involved in the case's deduction; in this story, Holmes's theory about the 'Hosmer Angel' character is the same, while Mycroft deduces that 'Hosmer Angel' is a fiction created by the mother and daughter to eliminate the step-father, only for Watson to reveal that 'Hosmer Angel' is actually a real person who was attacked and robbed on the way to his wedding, hospitalized, and eventually treated by Watson, who used his own detective skills to verify the man's identity.
Margaret Brown wrote: "Much as I admire Sherlock Holmes, I am always seized with impotent fury at reading the end of 'A Case of Identity'. What a patronizing arrogance to decide for her whether or not she could stand hearing the truth! Anyway, he was manifestly unethical to his client. She engaged him to find Hosmer Angel. He found Hosmer Angel. He should have given his client the information she wanted and let her decide what to do with it. (...) Anyway, what is this nonsense about the villain being beyond reach of the law? In British law of that time, a man could be sued for breach of promise. Even a bachelor who proposed to a woman with complete sincerity and then changed his mind could be sued. All the more so a married man who went through an elaborate charade and fallaciously courted his own daughter in law! Any half-decent lawyer could have broken him in court. Of course, the young woman might have chosen not to sue him - but Holmes should have left the choice to her. For me, this story is a dark blot on the otherwise admirable career of Sherlock Holmes".