Twelve episodes out of the twenty-nine made for this series no longer exist. Season 1 is almost complete with only one episode missing (Ticket to Terror), whilst Season 2 is almost completely missing with only two episodes surviving (Black Echo and A Sinister Sort of Service). In 2003, D for Destruction, the Season 1 finale episode, was recovered from the BBC in a mislabeled film can. It was previously an episode thought not to exist.
In the series' original pilot episode the part of Georgie was played by Ann Holloway. Although elements of the pilot were used in the existing opening instalment, there is currently no known copy of this untransmitted edition in existence.
In 2002, the long-running CBBC magazine show Blue Peter (1958) ran an eight part Victorian era drama called "Quest II: The Hunt for the Blue Stone". Gerald Harper appeared, whilst the mysterious whispering villain, whose face was concealed behind a mask, was tellingly called The Visage. The serial also tipped its hat to The Avengers (1961) and The Prisoner (1967).