Menou, a young Executioner, faces the daunting task of exterminating the Lost Ones, mysterious wanderers from the distant realm of 'Japan' who unwittingly unleash chaos. However, everything changes when she encounters Akari, a girl she finds impossible to kill. Determined to unravel Akari's secret of immortality, Menou embarks on a transformative journey with her unexpected companion, leading to profound changes that will alter the course of her life.