In a world where humanity faces an existential threat from mysterious entities known as "Unknown," three city-states—Tokyo, Kanagawa, and Chiba—serve as the last bastions of human civilization. These cities are protected by young individuals called "Children," who possess supernatural abilities granted by a phenomenon known as "World." The story unfolds across these three regions, each governed by its own set of leaders and strategies for combating the Unknown.

In Tokyo, the stoic and pragmatic Ichiya Suzaku leads the charge alongside his childhood friend, the cheerful and determined Canaria Utara. Their dynamic is marked by mutual respect and a shared determination to protect their city. Meanwhile, in Kanagawa, the confident and flamboyant Asuha Chigusa and her reserved yet fiercely loyal brother, Kasumi Chigusa, navigate their roles as leaders. Their relationship is complex, with Asuha often taking the spotlight while Kasumi supports her from the shadows. In Chiba, the enigmatic and aloof Hotaru Rindou and the energetic, battle-hungry Maihime Tenkawa form an unlikely duo, balancing each other's strengths and weaknesses as they defend their territory.

The narrative explores the Children's struggles against the Unknown, who are revealed to be more than mindless invaders. As the battles intensify, the characters uncover unsettling truths about their world, their abilities, and the nature of the Unknown. The story delves into themes of identity, loyalty, and the cost of survival, as the Children grapple with the moral complexities of their roles and the sacrifices required to protect humanity.

Throughout the series, the three city-states' leaders eventually cross paths, leading to alliances, conflicts, and revelations that challenge their understanding of the war they are fighting. The plot builds toward a climactic confrontation that forces the characters to confront their deepest fears and question the very foundations of their existence. The interplay between the characters' personal growth and the overarching conflict against the Unknown drives the narrative forward, creating a layered and engaging story.