 Dateline: the day after tomorrow. Spectrum agents Captain Scarlet and Captain Black are assigned to investigate the source of unexplained extraterrestrial signals which have been detected emanating from Mars. They discover a strange and fantastic alien city but when their Bison Alien Terrain Vehicle is approached by a glowing green sphere, Black assumes that they are under attack and launches missiles which destroy the city. Miraculously, the city reforms and its alien occupants, the Mysterons, are disgusted by the violence of the Earthmen, pledging to crush their world. Black is apparently killed but Scarlet is returned to Earth as the Mysterons  instrument of destruction...