An apocalyptic event has ravaged Japan, transforming it into an endless expanse of sand. Survivors fear a rusting disease that grips them, but their government blames Bisco Akaboshi, also known as the "Man-Eating Mushroom," for triggering the catastrophe with his release of mushroom spores. Bisco, however, sees things differently. He considers himself a "Mushroom Protector," driven to locate the fabled "Sabikui" mushroom, said to be a panacea for the rusting disease. On his quest, he is aided by his colossal crab friend, Akutagawa, and the empathetic Dr. Milo Nekoyanagi, who seeks a cure for his afflicted sibling. Despite facing public opposition, Bisco perseveres in his mission to restore the crumbling world.