When this episode was first aired, parts of Britain were suffering a power failure. It was also the day after President Kennedy was assassinated. Both of these factors meant the program was only seen by a disappointing 4.4 million people. When repeated a week later, it was watched by 6 million.
Susan remarks that Britain hasn't entered the decimal system yet. Britain would decimalize the pound in February 1971, after a few years of preparation. Amusingly, at the time this serial aired, pretty much all of the rest of the Commonwealth had either already decimalised, like South Africa, or was in the process of doing so, like Australia and NZ.
Susan's striped top belonged to Carole Ann Ford. It was part of an alternate costume she suggested for the character after it was decided to abandon the more adult, futuristic look of the unaired pilot. Ford's suggested outfit also included black leggings and boots, which were rejected for being too sexy, so jeans were worn instead. She would later wear the same striped top in The Great St. Trinian's Train Robbery (1966).
The first school scene was re-written to reduce the tension between Barbara and Ian. In the original script, Ian says: "When I've had a bad day, I come in here [the staff room], and I want to smash all the windows." Barbara retorts: "It hasn't been a bad day", and Ian remarks: "You're just naturally like that?" Barbara: "I hope not. I've had another kind of day. A very puzzling kind of day".
After Barbara and Ian leave Susan alone in the class room, the idea that Susan should do something mysterious was a late, unscripted inclusion. In the previously recorded original version, Susan doodled in the book. In this version, it was decided to abandon the doodling, and zoom in on the book instead. The design team asked for a book with an identifiable picture of Robespierre on the cover.