The panic caused by "UNKnown-LV4" in 199X plunged Japan into fear not once, but twice. But that fear has now become a thing of the past. Amid the devastating chaos, there were boys and girls who lost their families. They were raised like real siblings in a certain facility. But the facility, which was like their home, was shut down, and they were each separated - promising to meet again on this land. Time passed, and they reunited at the promised place, just as they had vowed. Along with the unwelcome "darkness".