The producers wanted Maureen O'Brien to dye her hair black to make her resemble Susan more. Maureen refused, and instead suggested the alternative of getting Carole Ann Ford back.
Koquillion was originally credited as being played by Sydney Wilson (a combination of the first name of Sydney Newman and the surname of Donald Wilson respectively) to preserve the "whodunnit" aspect of the story. This was the first instance of a pseudonym being used in a story to avoid giving away a plot surprise.
Ian Chesterton is seen wearing a wide tie with a triangle bottom, this marks the first time he doesn't wear his trade mark skinny tie with a square bottom.
The introduction of Vicki, and Maureen O'Brien's television acting debut. She had just come out of drama school.
During production, Carole Ann Ford visited the set to offer encouragement to her successor, Maureen O'Brien.