Planet Cray, a planet inhabited by humans and various species of monsters. A war has raged for centuries to counter the darkness that has disrupted the planet's peace. Intending to end the war, the Dragon Empire clan, led by Kai Toshiki, resolved to find the "KEY" to planet cray's peace, none other than Sendou Aichi. However, his meeting with Aichi made him unable to deny the fate that binds their lifes together.