Death lurks behind a mysterious orphanage where children are trained from a young age to become magical weapons of war. Fourteen-year-old Sheena Totsuki attends this facility, hating the fighting and longing for peace, but powerless to change the brutal reality around her. Her magical abilities are unremarkable, and she struggles to suppress her emotions, knowing that grieving fallen comrades is discouraged in this silent, regimented environment. On the night her roommate dies, Sheena encounters a strange girl covered in blood who smiles despite her horrific injuries. The next day, this girl transfers into Sheena’s class as Mimi Kagari, the rumored secret weapon of the school—an immortal child soldier who cannot die.

As newly assigned roommates, Sheena and Mimi grow closer, though their outlooks on war could not be more different. Sheena despises the violence, while Mimi, due to her immortality and childlike innocence, throws herself into deadly battles with a cheerful disposition and no hesitation to kill. Mimi’s healing factor allows her to regenerate from any wound, even coming back from a state where her body no longer retains human form. However, this immortality comes with a cost: she cannot age, both physically and mentally, and her existence is closely guarded by military authorities.

The story expands to include classmates like Lizzy Seiran, who has a strong sense of justice, and her gentle roommate Ali Maud, with whom she shares a deep romantic bond. Another key figure is Esta, an upperclassman who is first seen kissing her roommate, only for that roommate to die in battle shortly after, foreshadowing the tragedy inherent to this world. Haru, a student who wears a boy's uniform and suffers from a weak physical condition that prevents her from going outside in sunlight, struggles with her gender and romantic identity. The school nurse Fran, skilled in healing magic, looks after Mimi and administers a transformation potion that allows Mimi to advance to the next school year alongside Sheena.

As the conflict intensifies, Mimi’s status as a classified weapon puts her at odds with the authorities, especially when she discloses information to Sheena. Mimi throws herself into even more deadly clashes, surviving fatal wounds through a dark magic connected to love—the source of her immortality, which required a caster to sacrifice their life force to bring her back. This magic also manifests in the world’s healing system, where magic is transferred through lip contact, forcing characters to engage in intimate acts to save each other’s lives. Sheena, who has been physically maimed in battle and healed through these means, commits herself to protecting Mimi, even as the immortal girl puts national security at risk just by existing. When Sheena witnesses Mimi returned from the battlefield completely dismembered, she learns the full horror of what immortality means in a world of endless war. The series follows whether Sheena can harness the power of that love to rescue Mimi and help bring about peace, all while surrounded by classmates who face sudden death without warning, leaving no bodies behind as they dissolve into light upon dying.

An anime television series adaptation by studio Roll2 has been announced.