Due to British television's shift from 405-line technology to 625-line, in preparation for colour transmissions, going into effect for all BBC shows from 1 January 1968, it was long believed that the switch-over for "Doctor Who" from 405 lines to 625 came as of Episode 3 of this serial; however, upon the recovery of the other five episodes of the serial, it was discovered that Episodes 1 and 2 were in fact made at 625 lines prior to the official switchover. The now-disproved notion of the switch-over occurring at Episode 3 was most likely due to an error in documentation.