There is a scene where the Doctor and Jo are working on the TARDIS console in the Doctor's lab. A mistake by the Doctor causes another Doctor and Jo to briefly appear at the entrance to the lab. Originally the serial was to end with a scene where the Doctor and Jo went back to the lab, and saw their earlier selves working at the TARDIS console. However, director Paul Bernard refused to film it, saying "Once it's over, it's over". Terrance Dicks would later restore the scene in his novelisation of the story.
These episodes, apart from the two Doctor Who films Dr. Who and the Daleks (1965) & Daleks' Invasion Earth 2150 A.D. (1966) are the first time that Daleks had been seen in colour.
The same year, Aubrey Woods (The Controller) starred in Willy Wonka & the Chocolate Factory (1971) as Bill the sweet shop owner. Jon Pertwee had been offered to play Willy Wonka in the film, but turned it down due to his commitment on Doctor Who (1963).
This is the earliest Doctor Who (1963) serial for which all episodes remain on their original PAL 2" colour videotape.
Producer Barry Letts disliked the performance of Aubrey Woods, considering it too theatrical for television.