The Mechanoids made their only appearance in this story - usually referred to as "The Chase" - owing to the difficulties the men inside them were having in controlling them; they would not even fit through most doors.
The original script introduced the Fungoids on Aridius, as opposed to confining them solely to the Mechanus sequences.
This is the first instance where a doppelganger of the Doctor appears.
Edmond Warwick (the Robot Doctor) had previously appeared as the Doctor in The End of Tomorrow (1964) when William Hartnell was injured and necessitated a double; prior to that he'd appeared in The Keys of Marinus (1964). All three stories were written by Terry Nation.
Noticeable in towards the end, in the Dalek attack force are Dalek Props from Dr. Who and the Daleks (1965). These Daleks are easy to distinguish as they are more colourful than the silver and blue regular props, they are missing the rim section from the base and are missing the slats from the torso section. These are, in fact, the first appearance of the Movie Daleks as the feature film was yet to be released.