At the start of class, the death of Sheena Totsuki's roommate is announced casually, and the day continues as usual. In an orphanage that raises children as soldiers, death is a common occurrence. With their home country caught in a prolonged war that shows no signs of ending, the children are sent off to battle every day. Unlike the other orphans, Sheena is deeply troubled by the ongoing conflict and wishes for it to cease. On the night of her roommate's death, Sheena encounters a blood-covered girl who remains cheerful despite her condition. The next day, this girl appears as a new student in Sheena's class, introducing herself as Mimi Kagari. Initially thought to be a school legend, Mimi is rumored to be the orphanage's secret weapon—a girl who cannot die. Assigned as Sheena's new roommate, Mimi hopes to bond with her and adapt to school life. However, disturbed by Mimi's carefree attitude towards the war, Sheena struggles to accept their reality, even as their unusual friendship develops amidst the grim circumstances.