Location filming for this serial was hit by bizarre weather: one day snow, next rain, then sunshine. The script for the studio scenes was subsequently amended so this could be explained as freak weather conditions affecting the area.
The Master didn't appear in the original script. He was added by Barry Letts and Terrance Dicks, to the annoyance of Bob Baker, as he did not feel that the Master fit well in the established plot.
It was Terrance Dicks' idea that the interior of Axos should appear organic rather than man-made, and that Axos would drain the life out of the surrounding countryside.
The use of Axonite was originally to have been demonstrated using a rat instead of a toad.
This episode was watched by 7.3 million viewers on its original transmission.