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, each flying saucer is fueled by a green radioactive liquid, which leaks out from destroyed crafts as they crash. Through this, the fuel infects creatures, humans, and robots, creating a war between the monsters. The player acts as one of these monsters and battles against the rest in fictional cities across the globe and the remaining UFOs.