Sherlock's brother Mycroft (played by Charles Gray) took the place of Dr. Watson in this episode because Edward Hardwicke (Watson) was unavailable, as he was filming the motion picture Shadowlands (1993). It is the only episode of the series that does not include Dr. Watson.
Frank Finlay (Professor Coram / Sergius) previously played Inspector Lestrade in the Sherlock Holmes films A Study in Terror (1965) and Murder by Decree (1979) and Sir Arthur Conan Doyle in The Other Side (1992).
This was the first episode filmed in May 1993 for the 1994 series. Due to everybody being cast, it was learned that Edward Hardwicke would not be able to be available. During the week the writers re-wrote the script removing Dr Watson and placing Charles Gray, who just happen to be available, and giving him most of the lines and changing some of the attitude that Mycroft already existed. The lines was split between Mr Gray and some given to the Inspector. Hardwicke was available for the next filming session in two weeks time.
Sir Arthur Conan Doyle set the story in 1894 but this particular adaptation is set in the early 20th Century to allow the writer to flesh it out with the subplot about women's suffrage.
The picture above the fireplace that Holmes leans on is the illustration "Sherlock Holmes at Reichenbach Falls" by Sidney Paget from the story "The Final Problem"