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.

As Bees and ex-Bees alike gather in Yuusari, the rampage of the unstoppable Gaichuu Cabernet continues - and heart attacks only seem to make it more powerful. As it absorbs the attacks, Cabernet keeps morphing into stranger and more sinister forms. Meanwhile, a tragic accident triggers a different kind of transformation in Niche...