"The Adventure of the Devil's Foot" is one of the 56 Sherlock Holmes short stories written by British author Sir Arthur Conan Doyle. It is one of eight stories in the cycle collected in His Last Bow. Doyle ranked "The Adventure of the Devil's Foot" ninth in his list of his twelve favorite Holmes stories.
Due to Holmes' declining health, his doctor sends him and Dr John Watson to the countryside to convalesce. They rent a small cottage on the coast at Poldhu Cove in Cornwall, a region famous for its tempestuous seas and lonely moors dotted by the ruins of a lost civilization. but the holiday ends with a bizarre event. Through his interest in the ruins Holmes soon meets Mr Roundhay, the vicar of the nearby hamlet of Tredannick Wollas and an amateur archeologist, and Mortimer Tregennis, the vicar's boarder.
Shortly afterwards Tregennis and Roundhay come to Holmes at his cottage to inform Holmes of terrible event which occurred the night previous at Tredannick Wartha, the Tregennis family home. Mr Roundhay reports that Tregennis spent the previous night with his siblings Owen, George, and Brenda at their house playing whist, but left shortly after 10 o'clock. The next morning while out on an early walk he encountered a doctor who had been urgently called to Tredannick Wartha and who offered to take him with him. Upon arriving at the house he found his siblings still seated around the card table, the sister dead and the brothers completely insane, with all of their faces convulsed in horror. Mrs Porter, the housekeeper, reported nothing suspicious, and nothing was stolen or otherwise disturbed. Holmes, agreeing that it seems a very exceptional incident, agrees to look into the case.
Before starting off towards the house, however, Holmes questions Tregennis, who confirms most of the vicar's account and declares that it is certainly the work of a supernatural agent. Tregennis also reveals that he was estranged from his family over financial issues, though stating that they had been reconciled for some time. He also says that after seeing George looking through the window, he himself turned to see some "movement" outside, but let the matter pass as unimportant. The doctor who was summoned estimated that Brenda had been dead for at least six hours, and there were no signs of violence on the body. Both Tregennis and the doctor were nevertheless overcome by horror upon entering the room: in fact, the doctor had been so shocked by the sight of the family that he had nearly fainted himself. Holmes, satisfied with Tregennis' account and now more intrigued, sets off for the house without delay.
On the road to Tredannick Wartha the group pass a black carriage, within which Watson sees a man grinning insanely and gnashing his teeth. Tregennis tells him that the carriage contains his brothers, who are being taken to Helston.
Upon arriving at the house Holmes carefully inspects the garden and the flower-beds beneath the sitting room window, knocking over a watering pot and drenching everybody's feet in the process. Inside the house they are met by Mrs Porter the housekeeper, who tells Holmes she heard nothing the night before and that the family had been particularly happy lately. She had fainted in horror upon seeing the scene, but after recovering had sent a boy to fetch the doctor. Miss Tregennis had been laid on her bed upstairs, and four strong men had been needed to force the Tregennis brothers into the asylum carriage. After her interview, she announces her intention to leave that afternoon to rejoin her family in St. Ives.
Holmes and Watson venture upstairs to view Miss Tregennis' body, but soon descend downstairs to examine the sitting room. Holmes notices the ashes in the fire grate, and from Tregennis learns that they had lit a fire because it had been an unusually cold and damp night. Deciding there is nothing more to learn, Holmes and Watson return to their cottage.
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 person. 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.
Dr Leon Sterndale, the famous hunter and explorer, aborts his sailing from Plymouth after the vicar wired him (as the Tregennises are Sterndale cousins) with the tragic news. He asks Holmes what his suspicions are, and is displeased when Holmes will not voice them. After Stermdale leaves, Holmes follows him discreetly. The morning after Holmes comes back to his room, apparently none the wiser for following Sterndale, the vicar arrives in a panic with the news that Mortimer Tregennis has now died in the same way as his sister. The two men, along with Watson, rush to Mortimer’s room, and find it foul and stuffy, even though the window has been opened. A lamp is burning on the table beside the dead man. Holmes rushes about, examining many things. The upstairs window seems especially interesting. He also scrapes some ashes out of the lamp, and puts them in an envelope.
Holmes deduces how the victims died or went mad and why people present when the death rooms were first opened fainted or felt unwell in each case. He tests his hypothesis by buying a lamp like the one in Tregennis’s room, lighting it, and putting some of the collected "ashes" on the smoke guard. The smoke from this powder is so potent a poison that Holmes is immediately struck down. Watson is able to resist and drags Holmes out of the room just in time. It is clear to Holmes that Mortimer Tregennis poisoned on his siblings, but who killed Mortimer?
It is Dr Sterndale, who left physical evidence at the vicarage clearly implicating himself. Holmes confronts Sterndale, who explains that he loved Brenda for years (but had been unable to marry her because of the current marriage laws which prevented him from divorcing his wife even though she abandoned him years ago) and killed Mortimer in revenge for the cruel murder. The poison is called Radix pedis diaboli (“Devil’s-foot root” in Latin), which Sterndale collected from Africa as a curiosity. The toxic contents of the plant root are vaporized by heat and diffuse into the local atmosphere. He once explained to Mortimer what it was and what it was capable of, who then stole some to murder his siblings by throwing it on the fire just before he left. Mortimer thought Sterndale would be at sea before news reached Plymouth, but Sterndale recognized the poison’s effects from the vicar’s description of the tragedy and deduced right away what had happened.
Holmes’s sympathies in this matter lie with Sterndale, and he tells him to go back to his work in Africa.
"The Devil's Foot" served as the basis for a 1921 short film starring Eille Norwood as Sherlock Holmes, an episode of the 1965 television series Sherlock Holmes starring Douglas Wilmer, and a 1988 episode of The Return of Sherlock Holmes starring Jeremy Brett. Also, the 1944 film The Spider Woman is based on several of Doyle's Holmes stories, among them "The Devil's Foot".