Tatsumiya Island is a peaceful and tranquil island in a corner of Japan. Makabe Kazuki, a young boy on the island, is celebrating the spring of his fifteenth year with his classmate Toomi Maya and others. Blue skies, azure seas, school, and friends... life is the same as usual. But suddenly, the youngsters' daily lives are transformed by the sound of a beautiful voice echoing through the island. "Are you there?"
With the appearance of the mysterious life form, Festum, the children are drawn into a battle for the survival of the human race.