Originally the episode 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, the last episode was overrunning and director Paul Bernard decided to cut the scene, which he personally disliked. Terrance Dicks tried to persuade Bernard to put it back in, but Bernard refused and Barry Letts agreed that it should be cut. Dicks would later restore the scene in his novelisation of the story.
The actress playing Miss Paget was taken ill before the recording of Part 4, so her one line was given to extra Desmond Verini.
This episode was watched by 9.1 million viewers on its original transmission.
This episode takes place in England in the 22nd Century.
A line cut from the script established that all the Daleks infected with the Human Factor at the climax of The Evil Of The Daleks had been eradicated.