When Dr. Mortimer visits Sherlock Holmes and Dr. Watson in their rooms in Baker Street, he brings a centuries-old account of the death of the debauched and ruthless Sir Hugo Baskerville, allegedly killed by a diabolical hound. Dr. Mortimer’s friend and neighbor, Sir Charles Baskerville, recently died under circumstances that suggest that this ancient curse on the family persists. Mortimer is concerned for the safety of the Canadian heir, Sir Henry Baskerville, who is to arrive in London the next day en route to the Baskerville estate in Devon. Mortimer also describes the few neighbors on the moor, a group consisting of Mr. Stapleton, Miss Stapleton, Mr. Frankland, and Mr. Frankland’s daughter.
Arriving with Mortimer the next day, Sir Henry shows Holmes a note warning him to stay away from Baskerville Hall, and Holmes discovers evidence that his visitors were followed. Although Holmes is intrigued by the problem, he says that he has other obligations to honor first, so it is agreed that Watson will go to Baskerville Hall as companion, observer, and protector. From the Hall, Watson writes Holmes regularly and in detail about everything he learns and observes.
The moor, already forbidding at night, is now terrorized by Selden, the notorious murderer, who escaped from Princetown prison. Added to the presence of Selden and the possibility that the diabolical, night-stalking Hound of the Baskervilles has returned is the peculiar behavior of Barrymore and his wife. On the first night, Sir Henry and Watson hear a woman’s loud sobs. Later, Watson observes Barrymore stealthily placing a candle in a second floor window. By catching him in the act, Watson and Sir Henry force Barrymore and his wife to admit that he is signaling Selden, the murderer, who is Mrs. Barrymore’s younger brother. Sir Henry and Watson try unsuccessfully to catch Selden by following his answering light. In the process, Watson notices a tall, thin figure on a hill and deduces that this might be the person who warned Sir Henry against coming to Dartmoor. He determines to find this man and discover his intentions. Miss Stapleton, mistaking Watson for Sir Henry, urgently warns him to leave Devon. Sir Henry, meeting her later, is overwhelmed by her beauty and character and is on the point of declaring his love for her when Stapleton suddenly appears on the moor and castigates Sir Henry for daring to declare his affections. Stapleton follows this odd action by a visit during which he begs Sir Henry’s pardon and explains how accustomed he has become to his sister’s company. He invites Sir Henry to dinner and Sir Henry happily accepts.
During this time, Watson discovers that a young woman wrote a letter to Sir Charles asking him to meet her at the spot where the ghostly hound later chased him and frightened him to death. After speaking with her, he finds that she wrote the note at Stapleton’s urging. On his return to Baskerville Hall, Watson observes through Frankland’s telescope a boy running across the moor toward the remains of the Neolithic stone huts on the hillside. Since Barrymore and his wife have been supplying Selden, the supplies carried by this boy must be for the mysterious stranger Watson saw on the moor. Watson examines a number of stone ruins until he finds one containing evidence of habitation. There he sits and waits. At nightfall, he hears footsteps and a familiar voice. It is Holmes, who has been keeping watch on the moor while reading Watson’s reports. Scanning them for clues, he investigated Stapleton’s background, finding that he is in fact another heir to the Baskerville fortune and that the young woman living with him is his wife. As Holmes and Watson speak, a terrible scream announces the death of Selden, who, wearing Sir Henry’s cast-off clothes, fell fatally while pursued by the spectral hound. Laying a trap that night, Holmes, Watson, and Inspector Lestrade—just arrived from Scotland Yard—ambush and kill the hound as it stalked Sir Henry. Stapleton escapes into the Grimpen Mire, where he apparently takes a wrong step and sinks into the bog. Stapleton, it transpires, was the spy in London, and the woman posing as his sister sent the warning note.