With the problem of the black rocks finally resolved, Hiraku and his friends held a feast in the Great Tree Village to celebrate their hard work. Problems in other areas were also being resolved, and a peaceful time had returned to the village for the first time in a while. Meanwhile, a black cat that had appeared seemingly out of nowhere settled in the village, and Hiraku and his friends began to watch over it. On the other hand, the mountain elves worked on developing new tools, going through a process of trial and error. There were many setbacks, but their efforts gradually took shape, enriching the lives of the villagers.

As the flying castle approaches the Great Tree Village, Hiraku and his companions begin their operation to subdue it. Splitting into upper and lower groups, they infiltrate the castle, fighting off monsters while searching for a way to control it. Hiraku also joins the exploration, examining the dilapidated buildings and strange structures. As he continues his investigation, he notices an obviously unnatural dead end and mechanism, prompting him to step into a hidden space beyond. There, a new clue leading to the castle's secrets awaits.