The series suffered from the BBC's lack of a proper archiving policy until 1978. It was BBC policy before 1978 to wipe master tapes and reuse them for other programmes, hence saving money and storage space. Twelve episodes out of the twenty-nine made for this series are believed to 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.