You are the pilot of the Grandizer, docked aboard a space station that houses the Power Generation Unit (PGU) providing for the power needs of the planet. Aliens have attacked the space station, in order to destroy it, and the PGU that it houses. Your mission is to attack and destroy the alien mother ship, as well as any fighters that might intercept you, along the way. Collect booster units, left over from earlier unsuccessful attacks, to gain increases in speed, strength, and ammo.