After an apocalyptic catastrophe transforms Japan into a desolate wasteland plagued by a mysterious rusting disease, the government blames the chaos on Bisco Akaboshi, known as the 'Man-Eating Mushroom.' However, Bisco views himself as a 'Mushroom Protector' on a mission to locate the fabled 'Sabikui' mushroom, rumored to hold the key to curing the rusting affliction. Accompanied by his colossal crab companion, Akutagawa, and the compassionate physician Milo Nekoyanagi, who seeks a remedy for his ailing sister, Bisco defies societal prejudice as he strives to restore purity to the decaying world.