As with many a story, this one begins in space, with Space Japan being Space Attacked by some sort of giant Space Monster. In this particular case, it's Space Skynet. Of course, the only reasonable response to this threat is to make a giant humanoid robot that basically never actually walks (and instead flies?!) to attack Space Skynet's rampaging armies. At least you have some sweet tunes and majestic views to aid you in your journey!