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.

Lag returns from his training to find Amberground in chaos. The government is crumbling, revolution is underway and the sun itself is going out. The Letter Bees' last chance is to penetrate Akatsuki, the mysterious capital of Amberground and the source of the sun's power. But who will be the one to enter Akatsuki and forge a new world--Chico, who's prepared to make any sacrifice for her people, or Lag, who holds out the impossible hope that every life can still be saved?