Due to injury, William Hartnell is almost entirely absent from episode. He does briefly appear in a reprise from the previous episode and does a voice over for Edmond Warwick (who was only seen from behind, falling over). Acording to the BBC DVD commentary, Hartnell was injured while filming the battle at the Dalek saucer in episode three (the man carrying Hartnell down from the Dalek flying saucer dropped, head first into a metal camera stand). Warwick later appeared as the First Doctor's robot double in another Dalek story, The Chase.
In the original script, David and Susan were menaced by mutated humans in the sewers instead of crocodiles.
This episode takes place in London in the 2160s.
Wells' role was originally much smaller - he was freed from the Dalek shuttle by Ian and left to tend to other escapees.
This episode was watched by 11.9 million viewers on its original transmission.