The final appearance of Ian Chesterton (William Russell) and Barbara Wright (Jacqueline Hill). Jacqueline Hill later would appear as Lexa in the 4-episode series beginning with Meglos: Part One (1980), whilst Russell would reprise the role of Ian in a few Canon introductions for the VHS release of "The Crusade" and later for Big Finish, as well as appearing as Ian 57 years later in the BBC centenary anniversary which aired on 23rd October 2022.
The original script didn't end with Ian and Barbara leaving.
First appearance of Steven Taylor, although Peter Purves previously played a different character in the serial.
William Russell said that William Hartnell grew angry with both him and Jacqueline Hill, when they informed him they were leaving the series.
The only episode to feature Steven with Ian, Barbara, Vicki, and the Doctor.