Aki isn’t a brave little boy, but he’s dedicated nonetheless to safeguarding his sickly, mute friend Fuyu. They and their little gang live deep underground, in the concrete warrens to which survivors have fled following the eruption of Mt. Fuji. The community is in decline, however, as illness and despair take their toll. Fuyu dreams of the outside world, which he is too frail to ever reach. But what colour is the sky out there?