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 finds his long-lost hero Gauche Suede! But Gauche's heart is so depleted that he has no memory of his former self, and now goes by Noir the Marauder. Lag's only hope to recover the real Gauche is a special letter bullet that will convey Lag's heart to his friend. Lag's not sure how to go about writing the perfect letter, but his Letter Bee duties offer some good lessons in connecting hearts.