Episode Four of "The Tenth Planet" is indeed a landmark episode. Not only did the first Doctor (William Hartnell) defeat the Cybermen for the first time, but he then underwent a regeneration into the second Doctor (Patrick Troughton).
Only the few seconds of the regeneration are known to exist from Episode Four. This sequence is preserved in a transmission of Blue Peter from the early Seventies.
For some years I was grateful to Blue Peter for preserving it. However, I now understand that the reason that the whole episode does not exist is that after the tape was borrowed to record this short sequence, it was never returned.
As Blue Peter celebrates its 40th anniversary, there will be a reunion of cast and crew from this era. Is this an opportunity to recover this tape?
ANDREW J CHISHOLM