Sherlock Holmes and Dr. Watson visit Miss Helen Stoner at her step father's country manor at Stoke Moran. Holmes makes a careful and detailed study of the room in which Helen's sister Julia died, and where she has been asked to sleep now because some repairs are being carried out in her own room. Holmes' suspicions have been aroused because on examining the room from the outside he discovered that no repairs were necessary to Helen's room and that the repairs were only a pretext to make Helen sleep in the same room in which her sister Julia died:
"Pending the alterations, as I [Sherlock Holmes] understand. By the way, there does not seem to be any very pressing need for repairs at that end wall."
"There were none. I [Miss Helen] believe that it was an excuse to move me from my room."
"Ah! that is suggestive."
Holmes then examines Helen's room from inside and discovers that there is dummy bell cord hanging just above her bed and a ventilator which opens into the adjacent room which is Dr. Roylott's:
"Very strange!" muttered Holmes, pulling at the rope. "There are one or two very singular points about this room. For example, what a fool a builder must be to open a ventilator into another room, when, with the same trouble, he might have communicated with the outside air!"
"That is also quite modern," said the lady.
"Done about the same time as the bell-rope?" remarked Holmes.
"Yes, there were several little changes carried out about that time."
"They seem to have been of a most interesting character -- dummy bell-ropes, and ventilators which do not ventilate.
At the end of the story Holmes explains to us that the dummy bell rope and the ventilator were necessary for the poisonous snake to slither down and bite its victim who would be sleeping on the bed down below:
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.