Holmes and Dr. Watson find themselves at Poldhu in Cornwall one spring for the former's health, but the holiday ends with a bizarre event. Mr. Mortimer Tregennis, a local gentleman, and Mr. Roundhay, the local vicar, come to Holmes to report that Tregennis's two brothers have gone insane, and his sister has died. Tregennis had gone to visit them in their village ('Tredannick Wollas'), played whist with them, and then left. When he came back in the morning, he found them still sitting in their places at the table, the brothers, George and Owen, laughing and singing, and the sister, Brenda, dead. The housekeeper had discovered them in this state, and fainted. The vicar has not been to see them yet. Tregennis says that he remembers one brother looking through the window, and then he himself turned to see some "movement" outside. He declares that the horrific event is the work of the devil. Mortimer Tregennis was once estranged from his siblings by the matter of dividing the proceeds from the sale of the family business, but he insists that all was forgiven, although he still lives apart from them. The doctor who was summoned, reckoned that she had been dead for six hours. He also collapsed into a chair for a while after arriving.Holmes goes to the house in question and, apparently carelessly, kicks over a watering pot, soaking everyone's feet. The housekeeper tells Holmes that she heard nothing in the night, and that the family had been particularly happy and prosperous lately. Holmes observes the remains of a fire in the fireplace. Tregennis explains that it was a cold, damp night.Afterwards, Holmes lays the case out to Watson thus: Quite obviously, there is no point in attributing the tragedy to the Devil; therefore, what took place can only be the work of a human.Whatever happened to those people happened right after Tregennis left, for they had not moved and everything was in the same place;Mortimer Tregennis went swiftly back to the vicarage where he lives (a footprint sample was obtained in the watering pot "accident");The only suggestion of an explanation-the "movement"-comes from Mortimer Tregennis;Given the weather, anyone appearing at the window and doing something horrifying enough to instantly kill someone would have had to come right up to the window thus trampling the flowerbed, which is still intact;What on earth could this person at the window have done to cause such horror?None of this seems to make for an elementary case, but soon, new questions are raised