While Holmes and Dr. Watson are on vacation in Cornwall , the intrusion of a local vicar, Mr. Roundhay, makes sure that their break is cut short. Mr. Mortimer Tregennis, a local gentleman estranged from the rest of his family due to a family dispute, found his brothers under a strange derangement and his sister, Brenda, dead after a friendly visit. The housekeeper had found them and on seeing their state had fainted. Tregennis says he saw his brother looking out through the window and adds he had seen “movement” outside. He then attributes the bizarre event as the work of the devil. The ghastly look of horror on Brenda’s face is a complete mystery. The summoned doctor came to the conclusion she had been dead for six hours and he too collapsed into a chair after arrival.
After attending the affected residence, Holmes kicks over a watering pot; the action is in fact a deliberate accident. The feet of all get soaked. Holmes notes the remains of a fire. Tregennis explains it was a cold, damp night. New questions arise when Dr. Leon Sterndale, a famous hunter and explorer, makes a visit after hearing of the tragedy. He is played by Denis Quilley, who has an amazing voice. The Tregennis family members are distant cousins of Dr.Sterndale.
Soon after, the vicar delivers the news of the death of Mortimer Tregennis; it was in the same manner as his siblings. Rushing to the room of the dead man, Holmes and Watson find the air smells foul and stuffy despite an open window. In addition, a lamp is burning. Holmes scrapes half of the ash from the lamp, leaving the rest for the local police. It is clear he knows how the victims met their deaths. He tests his hypothesis and is pulled into a stupor of madness. In my opinion, this was done rather cheesily. The quick thinking of Watson who resists inhaling the poison saves him from near peril. It turns out burning of the powder was the key to solve other complications.
It turns out there were two guilty parties in this tale: one was motivated by greed and the other by love. The poison is called Radix pedix diaboli – Devil’s Foot in Latin. This is how the adventure, found in the story collection His Last Bow, derives its name.