Maten Densetsu: Senritsu no Ooparts

You find yourself in the middle of the Tokyo district Akihabara. But is this really Akihabara? The buildings have turned into ruins, and you wander around the deserted streets without any idea of how you could possibly be in such a place. A strange giant head appears in front of you, and defeats you in battle. When you wake up, you find out that Japan is floating in the air, and is attacked by a mysterious force that spawned countless demons who brought destruction to the cities. People are hiding among rubble and in underground passages. Some of them are fighting the demons, joining an organization called Self-Defense Forces. The hero's first objective is to find a way to his home district Shinjuku, where he will obtain more information about the power that threatens Japan, and discover mysterious artifacts from an unknown civilization, called ooparts (out-of-place artifacts).