Milo and Bisco head toward Touhoku in search of the legendary mushroom, Rust-Eater. After reuniting with their friend, the giant crab Actagawa, Milo struggles with his training to ride the crab, but ends up communicating with it thanks to his kindness. That night the two of them try to rest at the Nikkou War Memorial only to reunite with a jellyfish girl they met in Imihama.

Bisco is gravely wounded by the shot from Kurokawa. Milo leaves him behind to storm into enemy territory and rescue his captured friends, Pawoo and Jabi, but Kurokawa awaits with meticulously set traps. During their confrontation Kurokawa reveals his great ambition and the secret behind the Mushroom Keeper's persecution.