Plot

While convalescing in Cornwall, a depressive Holmes investigates the apparent death from apoplexy of a local woman and the unexplained sudden dementia of her two brothers.

Suffering from depression and nearing a breakdown, Holmes is accompanied by Watson to Cornwall for rest and recuperation as they tour the area's scenic coastal cliffs and Neolithic sites. The tranquility is interrupted by news that an unexplained tragedy has suddenly struck three siblings while playing cards in their home. The sister has apparently died from apoplexy, and her two brothers are suffering from acute dementia with expressions of fear and horror frozen on their faces. Showing great will power, Holmes discards his cocaine hypodermic and scrutinizes those involved in the case: an estranged third brother, the local vicar, and an enigmatic big game hunter and African explorer.—Gabe Taverney (duke1029@aol.com)

Sherlock Holmes is convalescing in Cornwall (accompanied by Dr Watson, of course) when three members of the same family are struck down with the same affliction. Brenda Tregennis is dead and her brothers Owen and George insane, found in the same room in their house. A fourth sibling, Mortimer, was also present for part of the evening and had a long-running dispute with his three siblings.

An ailing Sherlock Holmes travels under doctors orders to Cornwall for a period of rest and convalescence. Dr. Watson accompanies him intent on ensuring his friend follows orders but they are soon approached by the Reverend Roundhay to help solve a most perplexing incident. A young woman, Brenda Tregennis, apparently died of apoplexy while playing cards with her two brothers, George and Owen who are now suffering from some type of dementia. There are wild rumors going about the village including that the incident was a supernatural phenomenon. Holmes believes there is a far more grounded explanation and with the help of a surviving brother, Mortimer Tregennis, tries to determine who would want them dead.—garykmcd

Holmes (Jeremy Brett) and Dr. Watson (Edward Hardwicke) find themselves in Cornwall one spring for the former's health, but the holiday ends with a bizarre event. Mr. Mortimer Tregennis (Damien Thomas), a local gentleman, and Mr. Roundhay (Michael Aitkens), 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), dined whist with them, and then left. Tregennis did not stay with his siblings at the estate as there was some family dispute going on & instead stayed with the vicar at the vicarage.

When he came back in the morning, he found them still sitting in their places at the table, the brothers, George (Peter Shaw) and Owen (Norman Bowler), laughing and singing, and the sister, Brenda (Christine Collins), 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 during dinner before the tragedy, 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 in the same village. Tregennis tells Holmes that he kept his hopes alive of returning home some day.

The doctor who was summoned, reckoned that she had been dead for six hours, which places the time of death to very nearly after Tregnnis left the house. The doctor 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.

The case 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.

Dr. Leon Sterndale (Denis Quilley), the famous hunter and explorer, aborts his sailing from Plymouth to Africa after the vicar wired him (as the Tregennises are Sterndale cousins) with the tragic news. Sterndale 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.

Solution 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 can resist and drags Holmes out of the room just in time. It is clear to Holmes that Mortimer Tregennis poisoned his siblings to acquire the estate and the rest of the money that went to his siblings, but then 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), 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.