Set in the land of Amber Ground, a world where the sun never reaches, the story follows Lag Seeing, a young boy who works as a 'Letter Bee.' Letter Bees are elite couriers tasked with delivering letters and packages across dangerous, monster-infested territories. These couriers carry 'shindanju' (heart bullets) in their guns, which allow them to share the memories and feelings of the letter's sender with the recipient, ensuring that every heart is delivered to its destination. As Lag grows into his role, he searches for his mother and the legendary Letter Bee known as Gauche Suede, who once delivered him to his aunt. Along his journey, Lag uncovers deep-seated conspiracies regarding the government of Akatsuki and the true nature of the artificial sun that illuminates their world. Accompanied by his dingo, Niche, and her pet Steak, Lag faces off against Gaichuu (armored insects) and political forces that threaten the fragile peace of his society.