Helen Stoner comes to see Sherlock Holmes because she is terrified. She thinks someone may be trying to kill her, just as someone killed her twin sister Julia two years before. Sherlock Holmes and Dr. Watson meet with her in the waiting room on the ground floor at 221B Baker Street. Holmes tries to soothe her obvious agitation and urges her to sit closer to the fireplace because she is shivering.
"Pray draw up to it, and I shall order you a cup of hot coffee, for I observe that you are shivering.”
“It is not cold which makes me shiver,” said the woman in a low voice, changing her seat as requested.
“It is fear, Mr. Holmes. It is terror.” She raised her veil as she spoke, and we could see that she was indeed in a pitiable state of agitation, her face all drawn and grey, with restless frightened eyes, like those of some hunted animal.
Holmes takes her case because of her "pitiable state of agitation." She tells him she does not have any money to pay his fee. Holmes often works on a "pro bono" basis if a case intrigues him or if he feels sympathy for the client.
Helen tells him a long back story involving her violent stepfather. Her sister Julia died of unknown causes two years ago. Before her death she had told Helen she had been hearing a strange, low whistle at around three in the morning for the past few days. Dr. Roylott has now forced Helen to move into Julia's former bedroom, directly adjacent to his, by ordering some apparently unnecessary repairs to Helen's own room farther down the corridor. The first night Helen slept in Julia's bed she heard the strange, low whistle her sister had described. This is what has frightened her so badly. She was already upset about having to sleep in her sister's room and in her sister's bed. She came directly to see Sherlock Holmes as soon as it grew light.
Among all the other details she tells Holmes in her back story are the important facts that Julia was engaged to be married when she died and now Helen is engaged to be married within one or two months. It seems pretty obvious that Dr. Roylott would like to see his stepdaughters dead, because he is desperate for money and is legally obligated to pay either girl one-third of the income from their deceased mother's estate each year if and when she marries. But the biggest questions are: How could anyone have killed Julia when she was sleeping in a room with the door locked and the window covered by a bolted iron shutter? And how could anybody get to Helen in that same locked room?