After examining Helen Stoner's bedroom, Sherlock Holmes has concluded that Dr. Roylott is sending a snake through their communicating ventilator. He tells Watson later:
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.
Holmes asks Helen to place a light in her window when her stepfather has gone to bed for the night. He and Watson climb through her window and sit there in the dark in complete silence from about eleven p.m. until about three-thirty the next morning. Watson can tell the time from the church bell which rings in the distance every quarter-hour. After letting the two men inside, Helen had gone to sleep in her old bedroom. At about three o'clock Watson saw a brief gleam of light through the holes of the ventilator.
Someone in the next room had lit a dark-lantern.
Then about a half-hour later Watson heard something which he describes as
...a very gentle, soothing sound, like that of a small jet of steam escaping continually from a kettle.
When Holmes hears that sound he instantly strikes a match and lashes at the bell-pull with his cane. He drives the angry snake back up the bell-pull and through the ventilator, where it bites Dr. Roylott, who was not prepared to have his poisonous snake return so quickly at the sound of his whistle. He lets out a terrible scream. When Holmes and Watson enter his room they find him dead, with the snake, "the speckled band," wrapped around his head. Holmes expresses the moral of the story when he remarks:
Violence does, in truth, recoil upon the violent, and the schemer falls into the pit which he digs for another.
That resolves "The Adventure of the Speckled Band." Helen Stoner wishes to keep the matter a secret to protect the Roylott family name. The coroner's inquest concludes that Dr. Roylott was accidentally bitten by his swamp adder, which was part of a collection of exotic Indian animals he was studying.
Sherlock Holmes has not only solved the "locked-room mystery" of Julia Stoner's death two years earlier, but he has undoubtedly saved the life of her sister Helen. Not only that, but Helen will be happily married and will inherit the estate of Stoke Moran and all of the assets left by her deceased mother. Holmes had previously determined that the invested capital produced an annual income of approximately 750 pounds. It was this income which Dr. Roylott was trying to keep for himself by killing Julia when she became engaged and then by attempting to kill Helen when she became engaged two years later. According to the terms of her mother's will, Dr. Roylott would have been legally obligated to pay Julia one-third of that annual income if she had lived, and he would have been under the same legal obligation to pay Helen about 250 pounds a year unless he succeeded in murdering her. Roylott was deeply in debt. Stoke Moran was heavily mortgaged. He simply could not afford to pay either stepdaughter such large annual installments.