"The Adventure of the Copper Beeches", one of the 56 short Sherlock Holmes stories written by British author Sir Arthur Conan Doyle, and the last of the twelve collected in The Adventures of Sherlock Holmes. It was first published in Strand Magazine in June 1892.

Synopsis
Violet Hunter asks Holmes whether to accept a job as governess, with very strange conditions. She is enticed by the phenomenal salary which, as originally offered, is £100 a year, later increased to £120 when Miss Hunter balks at having to cut her long hair off, which is one of many peculiar provisos to which she must agree. The employer, Jephro Rucastle, seems pleasant enough, yet Miss Hunter obviously has her suspicions.
She announces to Holmes, after the raised salary offer, that she will take the job, and Holmes suggests that if he is needed, a telegram will bring him to Hampshire, where Mr. Rucastle's country estate, the Copper Beeches, is situated.
In about a fortnight, Holmes receives such a message, beseeching him to come and see her in Winchester. Once Holmes and Watson arrive, Miss Hunter tells them one of the most singular stories that they have ever heard. Mr. Rucastle would sometimes have Miss Hunter wear an electric blue dress and sit in the front room reading, with her back to the front window. She began to suspect that she was not supposed to see something outside the window, and a small mirror shard hidden in her handkerchief showed her that she was right: there was a man standing there on the road looking towards the house.
At another such session, Mr. Rucastle told a series of funny stories that made Miss Hunter laugh until she was quite weary. The one astonishing thing about this was that Mrs. Rucastle not only didn't laugh, but didn't even smile.
There were other unsavoury things about the household. The six-year-old child that she was supposed to look after was astonishingly cruel to small animals. The servants, Mr. and Mrs. Toller, were quite a sour pair. A great mastiff was kept on the property, and always kept hungry. It was let out to prowl the grounds at night and Miss Hunter was warned not to cross the threshold after dark. Also, Toller, who was quite often drunk, was the only one who had any influence over this brute.
There was also the odd discovery by Miss Hunter of her own tresses in a locked drawer. Upon checking her own luggage, however, they turned out to be another woman's, but identical in every way to Miss Hunter's, even to the unusual colour.
However, the most unsavoury thing of all about the household was the mystery wing. Miss Hunter had observed that there was a part of the house that did not seem to be used. The windows were either dirty or shuttered, and once she saw Mr. Rucastle coming out of the door leading into the wing looking most perturbed. Later, he explained that he used the rooms for his photography hobby, but Miss Hunter was not convinced.
She sneaked into the wing one evening, and had a truly frightening experience there. She thought she saw someone. Running out of the room, she found herself right in Mr. Rucastle's clutches. He seemed convinced that she had seen nothing of the prisoner in the forbidden wing. He also threatened to throw her to the mastiff if she ever went into the wing again.
It seems clear to Holmes that Miss Hunter has been hired to impersonate someone who looks very much like her, and he surmises quite reasonably that it is probably Mr. Rucastle's daughter by his first marriage, Alice. The story has been that she moved to Philadelphia. Holmes further surmises that the man watching the house is likely Alice's lover, or perhaps even her fiancé. The purpose of hiring Miss Hunter seems clear: she is to convince the man watching from the road that Alice is no longer interested in seeing him.
Holmes and Watson send Miss Hunter back to the Copper Beeches with certain instructions, and then arrive there in the evening with the intent of searching the forbidden wing. Holmes turns out to be right in most of his deductions, but the wing is empty. Someone has been there before him. Mr. Rucastle shows up as well, and in his anger, lets the mastiff loose, which has not been fed in two days (for Toller has been quite drunk). The mastiff attacks Rucastle. Even though he survives, Rucastle is a broken man, having paid hard for his evil intentions.
Rucastle's daughter has escaped with her fiance, and they marry soon after. Watson notes, at the end of the story, that Holmes appears to have been drawn to Miss Hunter. However, to his disappointment, Holmes does not show any interest in Miss Hunter after the mystery had been solved, which was the real force behind his feelings.