This episode sees the first appearance of Sherlock Holmes's brother Mycroft, played by Charles Gray. Mycroft became a recurring character throughout this series of adaptations of Conan Doyle's stories.
This episode has the first reference to The Diogenes Club. Dr Watson asks Holmes what the qualifications for membership of the club are. Holmes replies that they are "shyness and misanthropy."
This was the second time that Charles Gray played Mycroft Holmes. He previously did so in The Seven-Per-Cent Solution (1976).