The vacation of Sherlock Holmes and Dr. James Watson in the Cornish peninsula was cut short when Brenda Tregennis was found dead while her two brothers Owen and George sitting beside her body have lost their sanity and a great fear could be seen in their faces. Mr. Roundhay, the town’s vicar, and Mr. Mortimer Tregennis, brother of the victims and the last person to see his family alive and well, have asked Holmes’ help.
Holmes has found no signs of struggle inside or any traces of threat outside the house. But he has noticed the fireplace was used even it is spring season.
That afternoon Holmes and Watson were surprised to see Dr. Leon Sterndale, the great lion-hunter and African explorer, waiting for them in their cottage, and has shown great interest in the case. Leon said he has came back from Plymouth on his way to Africa when he received a letter from Mr. Roundhay about the tragedy and he would like to know who did these things to his cousins. When Holmes refused to give any details he left quickly but Holmes followed him.
The next morning Mortimer Tregennis was found dead in the same way of his sister’s death. Holmes, Watson and Mr. Roundhay rushed to the crime scene before the police came. Holmes has found important clues in the window and scraped a brownish powder from the lamp.
Holmes and Watson experimented on the powder and learned that when it burns produces a poisonous odor that can kill or cause people to see horrible images.
Dr. Leon Sterndale came to the place of Holmes upon Holmes’ request. Holmes boldly confronted him of his evidences and charged him of the murder of Mortimer. Leon confessed he killed him to avenge the death of Brenda the woman he have loved and has loved him too, the only reason he keeps coming to the peninsula. He got acquainted with her brothers because of her. Then one day Mortimer has come to his cottage and he has shown him his things from Africa including the poison Devil's Foot root, but he was unsuspicious that he will take some of it without his knowledge and use it against his own brothers and sister so he can have all their possession. When he has come back he decided to use the same poison to kill Mortimer so he can experience the pain, horror and death he inflicted upon his brothers and sister.
Holmes sympathetic of his reasons let him go to pursue his job in Africa.