Sherlock Holmes and
Dr. Watson find themselves at
Poldhu in
Cornwall one spring. The holiday ends when the local
vicar Mr. Roundhay and his lodger Mortimer Tregennis visit Holmes, asking for his assistance. The night before, Tregennis had gone to visit his three siblings, played
whist with them, and then left. The next morning, the
housekeeper found the trio still sitting in their places at the table; the brothers, George and Owen, had gone insane, and the sister, Brenda, was dead. Dr. Leon Sterndale, a famous hunter-explorer and a cousin of the Tregennises, aborts his sailing from
Plymouth after the vicar wires him with the tragic news. He asks Holmes what his suspicions are, and is displeased when Holmes will not voice them. The next morning, the vicar informs Holmes that Mortimer Tregennis has died in the same way as his sister. The two men, along with Watson, rush to Mortimer's room. A lamp is lit and smoking on the table beside the dead man. Holmes deduces that a poison, activated by combustion and affecting the cognitive functions, is the murder weapon; there were fires burning in both murder rooms, and people who entered them either felt ill or fainted. After a dangerous experiment in which he tests the residue of the poison on himself and is only just rescued by Watson, Holmes reveals that Mortimer Tregennis is guilty of the first crime and further deduces that Dr. Sterndale was Mortimer's murderer. Dr. Sterndale confesses that he held Mortimer at gunpoint and forced him to breathe the poison. It comes from a plant called the Devil’s-foot root. Sterndale once described the powder to Mortimer Tregennis, who later stole some from Sterndale's collection of African curiosities, then attempted to murder his siblings by throwing it on the fire just before he left. It is revealed that Sterndale was passionately in love with Brenda Tregennis, but was still married to and unable to divorce his first wife who had abandoned him years ago. Holmes' sympathies in this matter lie with Sterndale, and he tells him to go back to his work in Africa and never return. ==Publication history==