Gauche Suede is highly respected among the Letter Bees—a government organization that delivers written messages to even the most remote locations. However, the Amberground, the land Gauche must traverse, is blanketed in a constant twilight and home to monsters known as Gaichuu. On one precarious journey, Gauche meets Lag Seeing, a troubled young boy who has himself been sent as a letter.

Five years later, Lag joins the Letter Bee as well, inspired by his journey with Gauche. As he strives to become like his hero, Lag discovers the conflict and complexity at the heart of his world.