A scene cut from the script at the start of the episode 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 British Broadcasting Corporation 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".
Terry Nation developed more background than was made explicit onscreen in the finished production. The Voord were alien invaders who took advantage of the people of Marinus, rendered vulnerable by the pacifying effects of the Conscience. The Conscience was then deactivated to allow the Marinians to fight the Voord, and over the centuries, the time they spent on Marinus meant that the Voord, too, could now be affected by the machine. Therefore, Arbitan despatched agents to recover the keys which would reactivate the Conscience and allow him to finally defeat the invaders.
According to Jonathan Sothcott on the DVD commentary for Dr. Who and the Daleks (1965), Milton Subotsky considered adapting this serial into another movie.
Director John Gorrie was officially a member of the BBC's Plays department but was hired by producer Verity Lambert while he was on loan to the Serials department. He was reluctant to work on Doctor Who because he didn't like or understand science-fiction but Lambert insisted he was to do it.
This is the first episode of a six-part story commonly known as "The Keys of Marinus".