This story replaced a six-part script called Killers of the Dark, an ambitious adventure about a race of cat people with ties to Gallifrey. It included elements such as a forum the size of Wembley Stadium full of cat people. It was deemed impossible to achieve even on a film budget and abandoned.
Robert Holmes was asked to write this story. He declined, not wanting to return to Doctor Who (1963) so soon after he left. He did make contributions to the story, such as allowing the use of Borusa and splitting the story into two segments, the first four episodes being based around the Vardans and the final two episodes being based around the Sontarans.
The budget on the previous story, "Underworld", was so tight consideration was given to cancelling it and allotting the story's budget to this story.
John Arnatt was fourth choice for Borusa. Angus MacKay, who played the role in "The Deadly Assassin", was unavailable.
Fans voted this number 109 in a countdown of the 163 Doctor Who (1963) stories in Outpost Gallifrey's 40th anniversary poll in 2003.