Wendy Padbury liked her leather trouser suit so much that she bought it. The BBC's policies meant that she had to pay for it before filming started. Then, in the scene where Zoe has to crawl through a tunnel, the paint on the set turned out to be still wet.
The Doctor is absent from this episode in order to give Patrick Troughton a holiday during production. A double stands in for him when he is unconscious.
In the original script, the Martian spores were only intended to erupt after four weeks rather than almost instantaneously, and were initially destroyed by concentrated oxygen (possibly in a liquid form) instead of water.
This episode takes place on the Moon in the 21st Century.
This episode was watched by 7.1 million viewers on its original transmission.