This story was planned as a four-part serial, but was increased to five after the previous adventure, The Dominators, was reduced from six to five episodes. As a result, the first four episodes were only between 19 and 22 minutes in length and episode five was the shortest Doctor Who episode ever at just over 18 minutes. For this to happen, the first episode was cobbled together by the production team, making Peter Ling very unhappy.
This is the only episode to have no writer's credit.
As with most things in the land of fiction, the white robots are actually rooted in fiction outside of Doctor Who. They originally appeared in another BBC show called Out of the Unknown, although they were repainted for this story.
The white void that Zoe and Jamie are lost in after being lured out of the Tardis, was achieved by having the entire set walls and floor painted white. This effect was unintentionally greatly enhanced by the low picture quality of the TV broadcast, that blended the set floor and walls perfectly in to one white blurred background. This low grade picture quality version was then used instead for the original VHS releases of the story.
When released on DVD, as well as airings on the UK digital Channel; Horror. The actual high grade picture quality version was used, featuring more crisp clear imagery.
Unfortunately, this high definition made the painted set floor and walls all too obvious. Ruining the illusion the low definition quality did so well.