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 is reunited with his former boss, Largo Lloyd, now one of the leaders of the insurrection against the Amberground government. With Largo's help, Lag uncovers long-hidden secrets about his strange birth, the cryptic memories collected from children born on the Day of the Flicker, and the nature of Amberground itself. Meanwhile, Lag's partner Chico faces her own dark past...