Aside from "The Keys of Marinus", this was the only script Terry Nation wrote for the series that didn't feature the Daleks.
The first scene of main guest lead Guy Crayford was removed in post. It was also the first scene of Chief Technician Grierson. Dave Carter is still credited for part one despite his only scene being cut
This story had working titles of: The Kraals, The Kraal Invasion and The Enemy Within. A rumoured working title was Return to Sukkannan, but this was actually the title of a second storyline by Terry Nation which had been submitted to the Doctor Who (1963) production office at the same time, but was apparently abandoned.