In the coastal city of Otaru on the northern Japanese island of Hokkaido, high school student Shuji discovers that his shy and clumsy girlfriend, Chise, is not who she appears to be. Their relationship has only recently begun, with both struggling to express their true feelings, when an unexpected attack by enemy bombers turns their city into a warzone. Amid the chaos and destruction, Shuji witnesses a small, brilliant object obliterate the enemy fleet. Searching through the wreckage, he finds Chise, but she has been transformed. Her body is now fused with massive metal wings and advanced weaponry, making her the ultimate living weapon created by the Japan Self-Defense Forces.

Chise reveals that this transformation happened without her knowledge or consent, a process that has altered her body and stripped her of a normal heartbeat. As an unnamed global war intensifies, she becomes the Japanese militarys last and most powerful line of defense. The story follows the couple as they try to maintain a normal teenage romance against the backdrop of a collapsing world. Their dynamic is strained by Chise's involuntary and increasingly frequent call to battle, as well as Shuji's helplessness and horror at her role as a human instrument of mass destruction. The central conflict lies within Chise herself, who desperately clings to her fading humanity as the weapon's cells slowly overtake her body and mind.

Alongside the central couple, the narrative explores the lives of their friends and classmates, all grappling with the wars encroaching reality. A young man named Atsushi enlists in the military to protect the girl he loves, while other friends face the grief of losing loved ones in bombings or the isolation of having family members sent to the front lines. As Chise’s condition worsens, she loses the ability to speak and her mental state deteriorates, forcing Shuji to confront the tragic reality that saving his girlfriend or stopping the war is impossible. The only promise they can fulfill is to return to the place where they first shared a kiss, as the planet edges toward an apocalypse from which there is no escape.