Gash gets pulled away from his crew into a side mystery that doesn't reset anything you care about. The movie respects the main series enough to ask a real question: what happens to the demons nobody chose. It's a detour that earns its place because it's about someone left behind, not about power scaling or stretching the tournament arc thinner.

Gash and his friends set out for a summer picnic at Mt. Fuji, where they meet Kotoha, a girl carrying a white magical book whose prophecies come true. A message in the book says Gash's mother is waiting in a cave in the forest. In the cave they find a blond youth named Wiseman instead, and while trying to help him they head deeper and stumble on a passage to another world. There they encounter the Black Knight, the strongest warrior of that world, who believes Gash stole the 101st magical book and attacks them. Gash and Kiyomaro have just one day to find the real thief, or they will be trapped in this other world forever.