This show was affected by BBC's archive purge between the late 1960s and 1978. During that time, the BBC wiped the master tapes of shows they considered of no future value. The total number of episodes known to exist on film or videotape stands at twenty (and a half) out of 49. Additionally, visual excerpts from eight episodes are held, plus various off-air audio sequences from 16 installments.
Robot suits used in the series 2 episode "The Prophet" later appeared, with a new paint job, as the White Robots in "The Mind Robber", a 1968 story in the final monochrome series of Doctor Who (1963).
The first three series were devoted to science fiction stories. The fourth series, with a new producer, Alan Bromly, mixed SF episodes and supernatural horror episodes.
Watching an episode of this series in 1969 inspired George Harrison to write "I Me Mine".