In the land of Amberground, an artificial sun serves as the only source of light for the capital city of Akatsuki while an eternal night persists. The further away one is from the capital, the darker and more dangerous it is. In the surrounding areas of Yuusari and Yodaka exist enormous armored bug creatures called Gaichuu that prey on people's hearts. Despite this, it is the duty of Letter Bees, a special group of mail carriers, to deliver letters throughout Amberground while risking their lives battling the Gaichuu.

Twelve-year-old Lag Seeing aspires to become a Letter Bee like Gauche Suede after their fateful meeting five years ago, where Gauche "delivered" Lag himself as a letter sent by his missing mother. Determined to see Gauche again and uncover the truth behind his mother's disappearance, Lag begins his journey to become a Letter Bee.