The anime takes place in a world where humans are continuing to fight a war against an enemy called UNKNOWN. UNKNOWN invaded several decades ago, and children entered a cold sleep to escape the invasion. When the children awoke, they found themselves with supernatural powers. The children set up defense cities in Tokyo, Kanagawa, and Chiba in order to protect the country from the UNKNOWN, who appear through the Tokyo Bay Gate.
(Source: Anime News Network)