A plague-like wind sweeps across the parched, sandy landscape of post-apocalyptic Japan. Rust is all that left in its wake as it washes over both live and nonliving things. Mushroom spores are said to be the source of the world's terrible situation. Bisco Akaboshi, also known as the "Man-Eating Mushroom," is a sought criminal archer whose arrows cause mushrooms to sprout wherever they land. He is, in fact, a "Mushroom Protector," a person who spreads fungus to nourish the earth and restore it to its former state. Bisco travels across the wastelands of Japan with the crab Akutagawa and the young doctor Milo Nekoyanagi in pursuit of the famous "Sabikui"—a panacean mushroom reputed to eat all forms of rust.