At a quarter past seven in the morning, one day early in April 1883, Dr. Watson wakes to find Sherlock Holmes standing beside his bed. Holmes tells him that there is a young lady waiting in the sitting room, and her presence at such an early hour suggests that the case on which she has come to consult him is an important one, likely to be of interest to Watson. Watson dresses quickly and comes out to meet the lady, who is veiled, dressed in black, and shivering with terror.

Holmes tells her not to be afraid and then remarks that the lady has come up to London by train, after driving to the station in a dog-cart. He deduces these facts from the return ticket in her hand and the spatters of mud on her sleeve. The lady confirms that Holmes is correct and begs for his help and advice in a situation which has frightened her profoundly, though her fears and suspicions are vague. She introduces herself as Helen Stoner and says that she lives in Surrey with her stepfather, who is the last of an ancient family, the Roylotts of Stoke Moran.

The Roylotts were once wealthy, but the family fortune was wasted and gambled away over successive generations, and Miss Stoner’s stepfather qualified as a doctor and built up a successful medical practice in Calcutta. However, he beat his butler to death in a fit of rage and was forced to leave India after serving time in prison. While still in India, Dr. Roylott married Miss Stoner’s mother, who died eight years ago in a railway accident. She left her money, at least a thousand pounds a year, to Dr. Roylott as long as her twin daughters, Julia and Helen, lived with him. When the daughters married, they were to have an allowance of their own.

Dr. Roylott is a hot-tempered, quarrelsome character, who is unpopular with and feared by his neighbors. His only friends are the members of a band of “gipsies” (Romani people), whom he allows to stay in the grounds of Stoke Moran. He keeps a cheetah and a baboon, and because no servant will stay in the house, Helen does all the cooking and cleaning, with which her sister, Julia, used to help her until she died two years ago. On the day when she died, Julia complained to Helen that she had recently heard someone whistling in the middle of the night. Later that night, Helen was unable to sleep and heard a terrible scream coming from her sister’s bedroom. As she opened her door and ran down the passage, she heard a whistle, then a clanging sound. Julia collapsed on the floor and died, with the words: “O, my God! Helen! It was the band! The speckled band!”

The coroner’s investigation showed that Julia Stoner had certainly been alone when she died. The door of her bedroom was locked, the windows were shuttered and barred, and the floor and walls of the room were quite solid. The doctors examined her for traces of poison or marks of violence but found none. Now, two years later, Helen Stoner is to be married. The wing of the house in which she sleeps is undergoing repairs, meaning that she has moved into the bedroom in which her sister died. Last night, lying awake in bed, Helen heard the same low whistle which had been audible immediately before Julia died. As soon as the sun rose, she left the house and came to London to ask Holmes’s advice.

Holmes says that there is no time to lose, and he will go to Stoke Moran that afternoon. Miss Stoner departs, and Holmes hypothesizes that the Romani band who frequent the grounds may have been responsible for Julia Stoner’s death, though he admits that this is as yet an ill-formed idea. As he and Watson are discussing the case, the door is opened with sudden force by a huge man wearing a top hat and holding a hunting crop. He introduces himself as Dr. Grimesby Roylott and says that he is aware his stepdaughter has been to see Holmes. He demands to know what Miss Stoner said, calling Holmes a meddler and a busybody. Holmes, however, smiles politely and refuses to be drawn. Roylott seizes the poker, bends it into a curve, throws it into the fireplace, and storms out of the room. Holmes coolly retrieves the poker and bends it back into its former shape, remarking that his own grip is just as strong as Roylott’s.

Holmes spends the morning researching the precise terms of Mrs. Roylott’s will, which he finds would have left Dr. Roylott with very little if both girls had married. In the afternoon, he and Watson travel to Stoke Moran, where they meet Helen Stoner, who tells them that Dr. Roylott is still in London. Holmes examines the bedroom in which Miss Stoner now sleeps and notices a bell-rope, which is not connected to any bell, hanging from a hook beside a ventilator, which communicates with Dr. Roylott’s bedroom, rather than the outside of the house. Miss Stoner tells him that both the ventilator and the bell-rope were put in about two years ago.

Holmes and Watson arrange to spend the night in Miss Stoner’s room. They sit there in silence and without a light, since Dr. Roylott, who has returned and is occupying the next room, would be able to see this through the ventilator. After three in the morning, they see a gleam of light through the ventilator and hear a very soft sound. Homes jumps up and lashes out at the bell-rope with a cane. At the same time, Watson hears a low whistle, then a horrible cry. Holmes says that this means it is all over, and the two men go next door to Dr. Roylott’s room. They find him dead, with a speckled band wrapped around his head. This, Holmes tells Watson, is a swamp adder, the deadliest snake in India.

Holmes later explains to Watson that Dr. Roylott had killed Julia Stoner by releasing the swamp adder into her room through the ventilator. The adder then crawled down the bell-rope and bit the girl in the bed below. This might have failed several times, hence the whistling to recall the adder night after night, but eventually, he could be sure that the snake would bite the occupant of the room. Since Helen Stoner was now due to marry, Dr. Roylott had attempted to murder her in the same way, but Holmes had lashed out at the snake, driving it back through the ventilator to Dr. Roylott’s room. Its temper roused, the swamp adder bit Dr. Roylott, meaning that Holmes is responsible for his death, but Holmes remarks that this is not likely to weigh heavily on his conscience.