After making his debut in this episode, Colonel Lethbridge-Stewart (soon to be promoted and forever after known as the Brigadier) will go on to become Doctor Who's most frequently appearing supporting character. He will participate in 157 episodes (22 stories) and have individual adventures with the second, third, fourth, fifth and seventh Doctors.
Jamie: So you meant what you said, then? You're not going back to HQ?
Evans: No, I'd be more of an hindrance, I would. Anyway, I'm not supposed to be down here. I'm a driver, I am.
Jamie: Aye, there is another word I could use. Look, all you want to do is save your own skin.
Evans: Well, it's the only one I got.
The master copy of this episode was wiped in or shortly after September 1969. No copy survives in the BBC archives. In the 2013 iTunes release, the episode is reconstructed from a complete soundtrack and telesnap stills.
First appearance of Nicholas Courtney as Colonel Alistair Gordon Lethbridge-Stewart, who would later be promoted to Brigadier and join the regular cast.