It was Julia Stoner who first used the term "the speckled band." She died in great pain two years before her sister Helen comes to consult Sherlock Holmes. Helen tells Holmes and Watson:
"At first I thought that she had not recognised me, but as I bent over her she suddenly shrieked out in a voice which I shall never forget, ‘Oh, my God! Helen! It was the band! The speckled band!’ There was something else which she would fain have said, and she stabbed with her finger into the air in the direction of the doctor's room, but a fresh convulsion seized her and choked her words."
At the end of the story it is apparent that Julia had been trying to indicate that the thing she called "the speckled band" had come from the room next door, which was their stepfather's bedroom. He had been sending a poisonous snake into Julia's room through a ventilator. It crawled down a dummy bell- rope directly onto Julia's bed. It must have been doing this for four nights, because Julia told her sister that she had heard a mysterious whistling sound for the past three nights. Evidently the snake had been on the bed--or perhaps even under the covers with the sleeping girl--for three nights before it bit Julia on the fourth night. Dr. Roylott has trained the snake to come back up the bell-rope and through the ventilator at around three o'clock in the morning by blowing on a whistle and rewarding the snake with milk when it returns.
Roylott killed Julia to prevent her from getting married. He would have had to give her a substantial sum of money if she married, according to the terms of his deceased wife's will. When Helen Stoner becomes engaged, her stepfather manages to move her into Julia's old room, where he intends to kill her by the same means. However, Helen comes to consult Sherlock Holmes on the morning immediately after the first night she hears the whistle her sister had told her about two years earlier. This means that the snake was on the bed--or in the bed under the covers--for one night but did not bite Helen that light. When she heard the whistle it meant that the snake was leaving her bed and crawling back up the bell-rope, but she would not have seen it in the pitch-dark room.
Evidently the doctor could not be sure the speckled band would bite the sleeping stepdaughter when he sent it through the ventilator. He only knew that it would bite the girl eventually. The bell-rope led directly down to the bed and the bed could not be moved because it was bolted to the floor. Since the speckled band was a reptile from a tropical country, it would not try to escape from the room into the cold night air but might even--though the author does not say so--crawl under the covers and curl up beside the occupant for warmth. Probably the girl--either Julia or Helen--would not get bitten unless in her sleep she turned over right on top of the snake. This must have been what happened to Julia two years earlier. The snake would have bitten her through her nightgown, and there would have been no sign of a bite mark on her exposed flesh.
Holmes and Watson are hiding in Helen's room--Julia's former room--on the second night that Dr. Roylott sends the speckled band through the ventilator. The two men sit in the dark until three o'clock in the morning. The snake must have been on the bed or under the covers for several hours without their knowing it. Then Dr. Roylott summons it back with his whistle.
Suddenly there was the momentary gleam of a light up in the direction of the ventilator, which vanished immediately, but was succeeded by a strong smell of burning oil and heated metal. Someone in the next room had lit a dark-lantern. I heard a gentle sound of movement, and then all was silent once more, though the smell grew stronger. For half an hour I sat with straining ears. Then suddenly another sound became audible—a very gentle, soothing sound, like that of a small jet of steam escaping continually from a kettle. The instant that we heard it, Holmes sprang from the bed, struck a match, and lashed furiously with his cane at the bell-pull.
The snake was infuriated by being beaten and retaliated by biting and killing Dr. Roylott when it retreated through the ventilator into his room.