An apocalyptic event has devastated Japan, leaving behind only endless sandy plains, while a strange rusting disease terrifies the remnants of civilization. The government claims that this horrific state of the new world stems from mushroom spores spread by individuals like Bisco Akaboshi, who is labeled the "Man-Eating Mushroom." However, Bisco strongly disagrees. In truth, he is a "Mushroom Protector" determined to find the legendary "Sabikui" mushroom, which is said to be the ultimate cure for the rusting disease. He is joined on this quest by his giant crab, Akutagawa, and the kind doctor Milo Nekoyanagi, who is actively seeking a treatment for rust poisoning to help his sick sister. Despite the prejudice he faces from common folk, Bisco refuses to abandon his quest to purify the rotting world.