The story is set in Japan after a "Rusting Wind" has coroded everything. People live in fear of the rust that corrodes cities and life in general. A boy named Bisco Akaboshi from the despised Mushroom Guardian tribe embarks on a journey to obtain the "Rust-Eating" mushroom — a miracle drug that purifies all rust — to save his dying teacher. During his journey, Bisco encounters Miro Nekoyanagi, a good-looking young doctor at Imihama, and together they search for a countermeasure against the rust consuming Miro's beloved big sister.