This story had specially filmed clips added to it (called bookmarks) showing the 3 heroes talking amongst themselves about possibly revealing to Tremayne their secret. Thus the story could be repeated at any time without confusing people.
The 2010 DVD box set has these bookmarks clips to allow the viewer to view the first story as originally shown or with the bookmarks incorporated into the story.
The first episodes to be filmed were "The Dark Island" and "The Gunrunners". In terms of broadcast order, "The Dark Island" was the 11th episode, whilst "The Gun-runners" was the 29th. This meant that by the time the cast got round to filming the first episode "The Beginning", they already had a good measure of their characters.
Robert Farnon gets a musical director credit on "The Beginning", though his contribution extends only to the re-using of a theme tune Patrick McGoohan rejected for The Prisoner (1967).
"The Beginning" is the only one not to have a teaser before the opening credits.
"The Beginning" took four weeks to film, two more than the other 29 episodes.