When Helen Stoner comes to consult Sherlock Holmes she tells him an exceedingly long back story involving her mother, her stepfather Dr. Roylott, her sister Julia and herself, going all the way back to the time when she and her twin sister were two years old and living in India. Holmes usually seems impatient with clients and their stories, but he seems unusually patient and considerate with Helen. At one point she tells him:
Shortly after our return to England my mother died—she was killed eight years ago in a railway accident near Crewe. Dr. Roylott then abandoned his attempts to establish himself in practice in London and took us to live with him in the old ancestral house at Stoke Moran. The money which my mother had left was enough for all our wants, and there seemed to be no obstacle to our happiness.
The significance of Roylott returning to his estate at Stoke Moran is that he has his stepdaughters isolated and pretty much at his mercy. He could never have kept such dangerous animals while living in London, but at Stoke Moran he can indulge in any eccentricities that suit him. He keeps a baboon and a cheetah which are free to roam the grounds. An astute reader might guess that he has other animals from India which Helen does not know about.
Helen's mother left Dr. Roylott capital earning about 1000 pounds a year when she died. That was enough for him to stop trying to make a living as a physician and return to his estate at Stoke Moran. The only problem was that he was required under the terms of his dead wife's bequest to pay each stepdaughter a substantial annual sum if she married. Holmes later that day does some investigation and tells Watson:
The total income, which at the time of the wife's death was little short of £1100, is now, through the fall in agricultural prices, not more than £750. Each daughter can claim an income of £250, in case of marriage.
This obviously gives Dr. Roylott a strong motive to kill Julia when she becomes engaged and an equally strong motive to kill Helen after she announces her engagement to Percy Armitage. Roylott is a violent and desperate man. There seems no question that he was responsible for Julia's death two years earlier, but the question is how he managed it. Did he hire a "band" of gypsies to do the killing? If so, how could they have gotten into the apparently impregnable room where Julia slept, and where Helen was now sleeping? The story might be called a howdunit rather than a whodunit. It resembles Edgar Allan Poe's "The Murders in the Rue Morgue" in falling into the sub-category mystery genre of "the locked room murder mystery."