Holmes brings Watson’s pistol.
A woman named Helen Stoner comes to hire Holmes to say that her sister was killed and she is afraid she is next, because she is the heir to a will and has an evil stepfather. After hearing the story of Dr. Roylott (the stepfather), the inhabitant of Stoke Moran, Sherlock Holmes has the case pretty much figured out.
Before they even go to Stoke Moran they have an altercation with the man that turns somewhat violent. He is not a fan of Holmes, and tries to threaten him. Holmes decides to take precautions. Holmes is concerned about Dr. Roylott, because he knows he is devious and cunning and not above violence, and he asks Watson to bring a gun with him when they go to Stoke Moran.
“…I should be very much obliged if you would slip your revolver into your pocket. An Eley's No. 2 is an excellent argument with gentlemen who can twist steel pokers into knots. That and a tooth-brush are, I think, all that we need.”
Once at the house, they go to work. They sit up in the room by themselves with the assortment of the tools Holmes needs for the case other than the gun: a cane, a candle, and matches. That is all he needs to confirm his theory.
Holmes had brought up a long thin cane, and this he placed upon the bed beside him. By it he laid the box of matches and the stump of a candle. Then he turned down the lamp, and we were left in darkness.
He does indeed confirm it. When they are sitting there, watching the ventilator, they see the snake come out it. They follow it back to Dr. Roylott and find that he has been bitten. The case is solved, and the suspect was killed with his own murder weapon!
Holmes has excellent deductive skills, and is not afraid of danger. He does, however, prepare for it. He actually makes a comment during this case that he regrets putting Watson into danger! In the end, however, the evil doctor got his due, and they do not have to do a thing now—except deal with the snake!