In March 2019, in the seaside city of Shizuoka, a high school freshman named Shira carries the heavy trauma of his mother’s suicide. His childhood friend Anko harbors a crush on him, sensing his fragile state. Shira lives with a passive form of despair, feeling that he wouldn't mind if a great earthquake came and sank his entire town into the sea. In contrast, Anko desperately wants to take him to Tokyo so they can build a future together.

Their lukewarm, suspended days are shattered when Shira encounters a mysterious woman, his teacher Mutsumi. Mutsumi hides a strange secret: scales are growing on her body, and she is gradually transforming into something not entirely human. In a world that feels like it is ending, Shira makes a drastic choice. He decides he wants to live with Mutsumi, declaring that he doesn't care if they exist on land or at the bottom of the ocean, and willingly chooses the fate of becoming a fish alongside her. For Shira, who has been passively waiting for an end, this connection to Mutsumi feels like a newfound hope, though it is dangerously close to being a form of suicide.

As Shira disappears into this strange new reality with Mutsumi, those left behind are thrown into confusion and investigation. Worried about Shira, Anko teams up with a curious classmate named Izaki, who has been sniffing around Mutsumi's secrets, to track the couple down. Meanwhile, Wako, Shira’s father and a police detective, is investigating the disappearance of Mutsumi’s former homeroom teacher, Urai. As these separate threads begin to cross and connect, the personal drama escalates into a larger societal crisis.

The story expands beyond a forbidden romance into the realm of a full-blown apocalypse. A mysterious condition, referred to as fish transformation infection, begins to spread across the globe. Several months after Shira carries Mutsumi, whose symptoms are progressing, into the sea to escape, the world is transforming. Infected individuals form new religious communes, and a transformed individual named HAYASHI begins preaching that becoming a fish is humanity's only hope. When a ceremony is planned to forcibly infect Anko in front of a crowd, the personal quests of Shira, Mutsumi, and Anko collide with the fate of humanity itself, leading to a conclusion that answers the question of what it means to be reborn at the end of the world.