This episode was wiped by the BBC and no copy of it is known to exist. Off-air audio recordings by fans returned to the BBC and original shooting scripts were combined to recreate an animated version for the DVD release.
The first story to feature UNIT and Benton.
John Levene was originally cast as a Cyberman, as he had done in The Moonbase. He got the role of Corporal Benton when the performer originally cast was dismissed for chronic tardiness.
The characters of Professor Travers and his daughter Anne from The Abominable Snowmen and The Web of Fear (the latter only) were supposed to reappear this story. Due to disputes with those stories' writers Mervyn Haisman and Henry Lincoln, they were replaced with Professor Watkins and his niece Isobel.
This became the most expensive Doctor Who (1963) serial at the time it was made due to an unprecedented amount of location filming.