In the 21st century, planet Earth became uninhabitable, prompting a mass migration to Planet Zi, the native habitat of metallic life forms called Zoids. Years later, Zi is nearing its end, and its residents embark on a journey to return to Earth. In an effort to restore Earth's habitability, the migrants devise a "Zi-Forming" plan. However, the scheme fails, leaving Earth in chaos. Overrun by Zoids and shrouded in unstable weather, the migrants struggle to survive. Settlements of the Empire and the Republic attempt to unearth Zoids buried within the Earth to strengthen their respective armies. The story follows second-generation Earth citizens Leo Conrad and Buzz Cunningham, who chance upon a girl named Sally Land as she escapes from the Empire.