Nicholas Courtney suggested to Philip Hinchcliffe that the Brigadier be killed off in this story, as he had become aware that he would no longer be required in the series' new format. Hinchcliffe turned down the idea, feeling that it would be an inappropriate fate for a character that had been so popular.
This serial features the last appearance of Tom Baker in his original costume and coat from Robot: Part One (1974).
First appearance of the shape-shifting Zygons, who will appear in The Day of the Doctor (2013) and The Zygon Invasion (2015)/The Zygon Inversion (2015).
Fans voted this number 24 in a countdown of the 163 Doctor Who (1963) stories in Outpost Gallifrey's 40th anniversary poll in 2003.
The script originally placed a far greater emphasis on the Skarasen and the loch. Robert Holmes felt that the Zygons were more interesting characters, especially since they could interact with the main cast.