Menou is an Executioner for the Church, trained to assassinate the Lost Ones: people pulled from Japan into this world, each carrying a destructive Pure Concept. Her latest target is Akari, a cheerful girl whose Pure Concept is Time, so she instinctively rewinds her own death and no ordinary killing sticks. Unable to finish the job, Menou travels alongside Akari under the guise of an escort, quietly searching for a way to end a girl who cannot die.