Susan’s departure was the very first of any of the companions, or for that matter, the regular cast. This departure set a template for companion departures and thankfully it was a very good template - arguably one that the series has not consistently lived up to. It certainly wasn’t an abrupt departure or one that we couldn’t see coming as her departure story progressed. Even the Doctor notices that Susan is starting to listen to David rather than to her grandfather, and the classic “and don’t stop pick daisies” line in episode 6 is both hilarious and quite telling. It is arguably the polar opposite of how Leela’s departure for romantic reasons was handled some 14 years later in The Invasion of Time.

Interestingly, it is often forgotten that it actually wasn’t Susan’s choice to leave - it was the Doctor’s choice to leave without her and make her decision to stay with David for her. This of course led to one of the all-time greatest moments in Doctor Who history, a moment which has only increased in greatness, significance and stirring the heart-strings as time has gone by - the Doctor’s speech that was to be used in both The Five Doctors and An Adventure in Space and Time. “One day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and prove to me that I am not mistaken in mine.”

For that reason alone this is one of the best companion departures ever.