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 almost breaks the record time on his Letter Bee test, securing a job as a Letter Bee! But his celebration ends with unwelcome news: the record holder, Gauche Suede, is no longer a Letter Bee! Shocked, Lag seeks out Gauche's little sister Sylvette to learn exactly what happened. But Sylvette has no answers, only memories and a broken heart. Gauche would never abandon his sister like that! Could it be he lost his heart...or his life?!