The Television Companion Quotes

“The first episode of the story was remade between the third and the fourth as there was electronic interference on the tape of the original recording. (The episode was indeed remade, but the real reason was that talkback – i.e. the sound of instructions relayed to the studio floor from the control gallery – was picked up and clearly audible on the soundtrack of the original recording.)”
― David J. Howe, The Television Companion: Volume 1: The Unofficial and Unauthorised Guide to Doctor Who