Helen Stoner asks Sherlock Holmes for help because she believes her life is in danger from her stepfather, Dr. Roylott, after the mysterious death of her sister Julia. Holmes and Watson investigate at the country house where Helen lives with Dr. Roylott. During their night watch in Julia's old bedroom, Holmes spots an Indian snake enter through an air vent and sends it back to kill Dr. Roylott in his room, solving the mystery of how Dr. Roylott was endangering Helen.