Revealing Mistake: Bromley's dead primord body is seen to breathe.
In the parallel universe, Stahlman is called Stahlmann, according to the closing credits.
Sutton: It's marvellous, isn't it? The world's going up in flames and they're still playing at toy soldiers!
[About the walls around the drillhead]
Sutton: [They] could stand up to an atomic blast.
The Doctor: Compared to the forces that you people have unleashed, an atomic blast would be like a summer breeze.
The deleted scene (see below) has a date stamp for this episode. The parallel earth is destroyed on 23 July (although the year is never stated).
Series regulars Caroline John and Nicholas Courtney are credited as Section Leader Elizabeth Shaw and Brigade Leader Lethbridge Stewart respectively.
The video release includes an additional scene in which the characters listen to a radio announcer describing the nationwide disaster. This was cut from the original broadcast as it was felt that viewers would recognise the voice as Pertwee's. It was, however, shown in North American broadcasts.