In the land of Amberground, an artificial sun provides the only light for the capital city of Akatsuki, while eternal night endures elsewhere. The further one travels from the capital, the darker and more perilous the environment becomes. In the surrounding regions of Yuusari and Yodaka, enormous armored insect creatures known as Gaichuu exist, preying on people's hearts. Despite these dangers, it is the duty of the Letter Bees, a specialized group of mail carriers, to deliver correspondence throughout Amberground, risking their lives in battles against the Gaichuu. Twelve-year-old Lag Seeing aspires to become a Letter Bee like Gauche Suede, following their fateful meeting five years prior, during which Gauche "delivered" Lag himself as a letter sent by his missing mother. Determined to reunite with Gauche and uncover the truth behind his mother's disappearance, Lag embarks on his journey to become a Letter Bee.