This enmity begins when an expedition from Earth led by Captain Black discovers their base on Mars. Mistaking a scanner for a weapon, Black orders his men to open fire on the base, destroying it completely. But then a device appears from concealment and emits a beam of light, and the base reappears as if the attack had never taken place.
The base's occupants identify themselves as The Mysterons. They have the power to recreate an exact likeness of an object or person, but only after the original has been destroyed. Swearing revenge for the attack on their complex, they kill Captain Black and recreate him with a new personality as their loyal agent on Earth, recruiting other agents and vehicles in a similar fashion. But their attempt to recreate Captain Scarlet himself fails when his duplicate is wounded and regains his original personality. The Mysterons themselves are never seen. They broadcast their threats by radio, and are only ever represented visually by a pair of glowing circles (suggestive of eyes) that are projected onto the scene.
From a logical point of view the Mysteron's hostility to Earth seems to make no sense. If they were able to recreate their base, why were they so upset by its destruction, and why do they continually ignore Earth's gestures of reconciliation?
According to Gerry Anderson, the answer is that the Mysterons are actually computers that were left behind on Mars by visiting aliens. This would seem to explain their lack of compassion, but why do they still ignore the logical alternative to conflict? Perhaps their logic circuits were damaged in the initial attack....