Trivia: There were a lot of changes to the show throughout the second season. Not only was Virgil "re-cast" with both a new head and voice actor (Jeremy Wilkin), but smaller, more lifelike puppet hands were crafted on the marionettes, and the inside of Tracy Island and the Thunderbird crafts were completely redone.
Trivia: Throughout the second series there were three 'guest' voice artists: John Tate, Paul Maxwell and Charles Tingwell. Tate had previously voiced Solarnaut Camp in the episode 'Sun Probe' in the first series, whilst Maxwell and Tingwell would appear in the next Gerry Anderson creation 'Captain Scarlet & The Mysterons' as the voices of Captain Grey and Dr Fawn, respectively. None of these artists were credited in the end titles.