Gerry Anderson has stated that the intended backstory for the Mysterons was that they were computer intelligences from another galaxy who had established a computer complex on Mars. Even though this explanation was never stated on screen, nothing was ever said to contradict it. The original, never-used, plan for the end credits was to show computer circuit boards. In the end, paintings of Captain Scarlet in jeopardy were used instead for the end credits. During the programme, the Mysterons were never shown and were often assumed to be invisible. However, in TV Tornado, the precursor to TV Century 21, the Mysterons were portrayed as physical beings. The Mysterons comic strips told the battle with Spectrum from the point of view of the Mysterons, and in an episode from September 1967, the Mysterons gained physical form. In fact, this strip showed the Mysterons as they looked originally. Presumably, they downloaded their consciousnesses into computers afterwards. Other strips showed a more robotic form for the Mysterons, further adding to the impression that they were artificial lifeforms.