- Creator Backlash: Carole Ann Ford was displeased with the portrayal of Susan in the serial, as she felt that she was written like a child, describing her character as "pathetic".
- Wag the Director: A rare instance of this being done by the production designer rather than an actor. Director John Gorrie was originally just going to play a "splash" sound to signal that one of the Voords had fallen into an acid pit in the first story, but the designer, Raymond Cusick persuaded him that it wouldn't be clear enough what happened, and that there needed to be an actual effects shot. After seeing the less-than-convincing result, however, Cusick admitted that he should have left well enough alone.
- What Could Have Been:
- A scene cut from the script revealed that the reason the Doctor and Susan had been on Earth in 1963 at the start of the series was because the Doctor had visited the BBC to get help repairing the colour scanner in the TARDIS, which was showing only monochrome images. He had been in such a bad mood upon his return to the TARDIS because the BBC had been infernally secretive!
- Originally, the Doctor accompanied Susan and Sabetha in Episode Four (and his ring would have been found in Vasor's hut along with the travel dials and Conscience keys). This was changed to give William Hartnell a holiday.
- According to Jonathan Sothcott on the DVD Commentary for Dr. Who and the Daleks, Milton Subotsky considered adapting this serial into another movie.
- Working Title: Planet Marinus.
Trivia / Doctor Who S1 E5 "The Keys of Marinus"