               The Adventure of the Devil's Foot
    He got a kind of rare drug unawares in Africa. While burning, this drug would make people produce a strong and horrible vision and frighten them to death eventually. The only person he had told about the drug was Mr. Tregennis. He found Mr. Tregennis had stolen some of it while he was off guard. When the explorer learned of the murder case, he concluded it must be done by Mr. Tregennis: he killed his sister, drove his brothers mad, thus he was the sole guardian of his whole family's property. This was a typical case of killing for money. Mr. Tregennis' sister was his lover. She was the woman that he loved best in his life. The explorer was determined to take revenge for his sweetheart. Therefore, in an early morning he sneaked into Mr. Tregennis' residence, forced him to speak out how he murdered his brothers and sister, and then killed him in the same way.
    Hearing the explorer's narration, Holmes narrowed his eyes and told Watson: "I've never loved. But I would also revenge if my loved woman had met with such an end. Mr. Tregennis really deserved the punishment!" 
