The three most important clues are all seen by Sherlock Holmes in the room at Stoke Moran currently being occupied by Helen Stoner, which had been her sister Julia's room when she died two years ago. Holmes notices the ventilator between Helen's room and her stepfather Dr. Roylott's room directly next- door. He also discovers that the bell-rope beside Helen's bed is a dummy and that the bed itself is fastened to the floor with clamps. These three clues suggest an answer to the big question in this so-called "locked-room murder mystery": How could Julia have been murdered in a room with the door locked and the window covered with bolted iron shutters? After the the case has been solved and Dr. Roylott is dead, having been bitten by his own snake, Holmes explains his reasoning to Dr. 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.
Holmes puts his theory to the test by staying in Helen's bedroom overnight, accompanied by Watson. The doctor sends the trained Indian swamp adder through the ventilator in the middle of the night while the two men are waiting silently in the dark. They are not aware that the snake is on the bed until Roylott summons it back with a low whistle at around three-thirty in the morning. When Holmes hears the whistle, he lights a candle and begins beating the snake with his cane as it is climbing back up the bell-rope. This enrages the snake and causes it to bite Dr. Roylott who is unprepared for its sudden return through the ventilator. He dies instantly.
Thus Holmes solves the two-year-old mystery of Julia Stoner's death and saves his client Helen Stoner from the same fate.