Jeremy Brett became too ill to shoot this episode, therefore a small segment at the start of the episode was shot featuring Brett explaining his absence. Charles Gray starred as Mycroft Holmes yet again and took Sherlock's place. Jeremy Brett his illness not withstanding also did a shot at the end of the episode were he praises his brother's success.
Adapted from two Conan Doyle stories: the title story ("The Mazarin Stone") and "The Three Garridebs".
Although the part of Nathan Garrideb was played by Richard Caldicot, at least one British TV guide (the established and renowned Radio Times) listed Sebastian Shaw as playing the part, which indicates Shaw may have been cast at an earlier stage.
Owing to the illness of lead actor Jeremy Brett, this episode had Sherlock's brother Mycroft (played by Charles Gray) written in to take Sherlock's place.
Helen Ryan plays the Princess of Wales but had already appeared in a previous episode, The Norwood Builder, as Mrs McFarlane.