The plot is set in the 1950s where a fleet of alien flying saucer warships invade the Earth, causing massive damage. The scientists of the world's nations manage to create a series of secret weapons, which, when activated, let loose shock waves that short-circuit the saucers and cause them to crash. Unfortunately, the flying saucers were all fueled by a green radioactive liquid, which leaked out as they crashed. Through this, the fuel infects animals, humans, and robots, turning them into giant mutant monsters, creating a war for supremacy among them. The player acts as one of these monsters and battles against the rest in fictional cities across the globe and the remaining UFOs.