The Spectrum team must once again face the inscrutable alien enemy, the Mysterons, in a battle for the very existence of Earth. Leading the fight is Captain Scarlet. On a mission to Mars, Scarlet and Captain Black encounter an alien city. Under the impression that they are under attack, Black launches a missile into the city destroying it. However, the city miraculously reassembles and a counter attack is launched and Black is killed. Back on Earth, Black is resurrected to become the Mysterons' agent of destruction and Scarlet is transformed into an indestructible being. Both men are now locked into a battle for the very existence of the human race.