Amberground is locked in darkness. A man-made star casts only a dim light over the land. The pitch-black wilderness is infested with Gaichuu--colossal insects with metal exoskeletons. The Gaichuu make travel between the cities of Amberground extremely dangerous. But thankfully the Letter Bees, a brave corps of messengers, risk their lives in order to keep the hearts of Amberground connected.

Gauche Suede is on his last delivery before a big promotion. In the outskirts of Yodaka, the darkest area of Amberground, Gauche is surprised to find that the package is a young boy named Lag Seeing. Lag had been traumatized by his mother's abduction and is due to be delivered to his aunt. In this remote area rife with Gaichuu, Lag and Gauche face a dangerous journey that inspires Lag to become a Letter Bee.