have her move to a room next to his own. It turns out that he has arranged to have a ventilation shaft constructed between the two rooms and has placed a rope in Helen’s end of the shaft.  He says the rope is a bell pull to summon a servant, but it is actually just tied to a hook.

Roylott’s plan was to be in his room and to put a swamp adder, which Holmes says is the deadliest snake in India, into the shaft and force it towards Helen’s room.  It will go down the rope to her bed (the rope touches the bed) and will bite her.  She will die in seconds.

Holmes, however, figures out what is going on and foils Roylott.  Roylott ends up getting killed by the same snake that me meant to use to kill Helen. 

When Helen Stoner comes to see Sherlock Holmes at Baker Street in the early morning, she tells him, significantly:

“When Dr. Roylott was in India he married my mother, Mrs. Stoner....My sister Julia and I were twins, and we were only two years old at the time of my mother's re-marriage. She had a considerable sum of money—not less than £1000 a year—and this she bequeathed to Dr. Roylott entirely while we resided with him, with a provision that a certain annual sum should be allowed to each of us in the event of our marriage."

A bit further on in her long back story, Helen tells the detective and his friend Dr. Watson about her and her sister's unhappy lives at Stoke Moran, but adds:

"We had, however, an aunt, my mother's maiden sister...and we were occasionally allowed to pay short visits at this lady's house. Julia went there at Christmas two years ago, and met there a half-pay major of marines, to whom she became engaged....but within a fortnight of the day which had been fixed for the wedding, the terrible event occurred which has deprived me of my only companion.” 

Before Holmes and Watson go down to Stoke Moran to examine the premises, Holmes does some research in the civil records and learns that the deceased mother's will provided that the girls' stepfather would be legally obligated to pay either of them one-third of the proceeds of the capital she had left him if either girl married. Holmes naturally suspects that Dr. Roylott must have been responsible for Julia's death two years ago to spare himself from having to pay her such a large sum annually once she was married. Holmes ascertains that:

"The total income, which at the time of the wife's death was little short of £1100, is now...not more than £750. Each daughter can claim an income of £250, in case of marriage. It is evident, therefore, that if both girls had married, this beauty would have had a mere pittance, while even one of them would cripple him to a very serious extent." 

Dr. Roylott would have a strong motive to kill Julia in order to avoid paying her and her husband £250 a year indefinitely. Now Helen Stoner has become engaged to be married within two or three months. Dr. Roylott has found an excuse for moving Helen into Julia's former room, which is right next to his own. He pretends that some repair work was necessary on Helen's own room, which is separated from Roylott's by Julia's. Evidently her stepfather wanted her to be sleeping in Julia's old room for some reason. The main mystery in "The Adventure of the Speckled Band" is how Dr. Roylott could have killed Julia when she was sleeping in a locked room which seemed impenetrable from the outside.

Holmes and Watson travel down to Stoke Moran and hire a trap at Leatherhead Station to take them out to Roylott's crumbling country mansion. There Holmes examines the room where Julia died two years ago and then examines Dr. Roylott's room next door. Holmes observes that there is a ventilator between the two rooms. In Helen's room there is a dummy bell-rope by the bed. The bed cannot be moved because it has been bolted to the floor. He and Watson stay in Julia's room that same night, and Holmes drives the "speckled band" back up the bell-rope and through the ventilator, where it bites Dr. Roylott and kills him instantly. Later Holmes explains his deductions to Watson:

"My attention was speedily drawn, as I have already remarked to you, to this ventilator, and to the bell-rope which hung down to the bed. The discovery that this was a dummy, and that the bed was clamped to the floor, instantly gave rise to the suspicion that the rope was there as a bridge for something passing through the hole and coming to the bed. The idea of a snake instantly occurred to me, and when I coupled it with my knowledge that the doctor was furnished with a supply of creatures from India, I felt that I was probably on the right track."

So Sherlock Holmes not only saves the life of his client Helen Stoner but solves the two-year-old mystery of the death of her sister Julia. Dr. Roylott had only one motive for murdering Julia with his poisonous snake. He was in desperate financial trouble and simply could not afford to pay Julia £250 a year out of his dwindling  assets. By killing her, he solved that problem. But then Helen became engaged and he had the same problem all over again. He moved Helen into the room next to his and was planning to kill her by the same means. But she became terrified on the second night when at around three o'clock she heard the same low whistle which her sister had described to her shortly before she died an agonizing death. Helen waited until daybreak and then hurried to London to consult Sherlock Holmes, whom she had heard of from a friend.

There were several ways in which Dr. Roylott's Eastern training helped him in killing Julia Stoner and getting away with murder. The main way is referred to specifically in the text. Sherlock Holmes explains it to Dr. Watson after the case is resolved and they are traveling back to London the next day.

"The idea of using a form of poison which could not possibly be discovered by any chemical test was just such a one as would occur to a clever and ruthless man who had had an Eastern training. The rapidity with which such a poison would take effect would also, from his point of view, be an advantage. It would be a sharp-eyed coroner, indeed, who could distinguish the two little dark punctures which would show where the poison fangs had done their work."

The rapidity with which the snake's poison took effect prevented Julia from warning her sister Helen that she had been bitten by a snake and that their stepfather was responsible for sending it into her bedroom. Julia was delirious and could only mumble something about a "speckled band" and point to Roylott's room. Then she died. 

"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."

Roylott's Eastern training has also taught him how to handle poisonous snakes. No doubt he had spent time studying the cobra in India, but that snake would have been too big for his purposes. It could not have slipped through a hole in the ventilator, and furthermore it's venom is probably detectable in an autopsy. Roylott knew how to handle the Indian swamp adder safely. The only reason he was bitten and killed by his own snake was that he wasn't prepared for its sudden return after being beaten by Holmes' cane, nor was he expecting it to be in such an aggressive temper.

Finally, Dr. Roylott's Eastern training had taught him how to train his snake to return to his room when he blew a soft whistle and rewarded it with milk before locking it back up in his steel safe. It was the whistle that had warned Helen of potential danger. She remembered that her sister Julia had told her of hearing such a whistle for three nights in a row before the night on which she died. Helen only waited for daybreak and then went directly to London to ask Sherlock Holmes for advice and assistance.

During his summing up of the case, Holmes tells his good friend Watson:

When a doctor does go wrong he is the first of criminals. He has nerve and he has knowledge.