“Who’s to know?”

The 1972 series of Doctor Who was the first during which no actor or character arrived or departed. It was also the first series of Doctor Who entirely written by people who had written for the show before. 

These facts could be considered, if one were feeling churlish, to be signs of complacency or stagnation. But in this instance what they demonstrate is stability. A stability had been hard earned by the behind-the-scenes diarchy of producer Barry Letts and script editor Terrance Dicks, who had been working together for a little under two years when the first of 1972’s stories started shooting in September 1971.