The snake comes through the ventilator and bites its victims, killing them with poison.
In this story, the client is Helen Stoner. She hires Sherlock Holmes because she fears for her life. Her twin sister died under mysterious circumstances, saying only something about a “speckled band.” She was about to get married and Helen wants to get married but she is afraid to because she has been hearing strange sounds!
Imagine, then, my thrill of terror when last night, as I lay awake, thinking over her terrible fate, I suddenly heard in the silence of the night the low whistle which had been the herald of her own death.
Holmes and Watson go to the country estate where Helen is staying with Sir Grimesby Roylot, a band of gypsies, and a bunch of wild animals he keeps as sort of pets. There are so many red herrings in this story, between the gypsies and the animals, it is hard to keep them straight, so it is not difficult to imagine that even the venerable Sherlock Holmes got off track a bit. However, he figures it out.
Holmes deduces that the “speckled band” is the snake. He lights the match to see it, and hears the hiss of the snake and the banging as it goes through the ventilator.
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. Then I thought of the whistle. Of course he must recall the snake before the morning light revealed it to the victim.
Roylot is a kind of an evil genius. He somehow figured out how to train the snake to go through the vent to kill his victims. Holmes considers him particularly despicable because he is a doctor, and doctors are not supposed to hurt people (especially not stepdaughters). As with most cases, it is all about money. If they marry, he gets less.
In this classic mystery story, you really have to pay attention to solve the mystery before Holmes does, or as Holmes does. Will the reader understand the clue of the whistle and the snake, without it being pointed out? Maybe not, but as soon as it is, it seems obvious! That's what makes a good mystery.