The Doctor is absent from this episode in order to give William Hartnell a holiday during production.
Originally, The Doctor accompanied Susan and Sabetha in this episode and his ring would have been found in Vasor's hut along with the travel dials and Conscience keys.
It was David Whitaker who suggested a nordic environment to contrast with the jungle setting of the previous installment.
Despite the scene where Vasor visibly attempts to rape Barbara, this story's VHS release was rated U.
This was the first of two William Hartnell serials that Francis De Wolff has a key role in, the other being "The Myth Makers".
