In the 21st century, life on planet Earth became impossible, and people were forced to migrate to Planet Zi where metallic life forms called Zoids thrived. After many years, the inhabitants of Zi were forced to migrate back to Earth as their home faced its demise. The migrants tried to restore Earth's habitability through a "Zi-Forming" plan, but the plan failed, leaving Earth overrun by Zoids and plagued by unpredictable weather conditions. The migrants struggled to survive, with the Empire and Republic settlements attempting to strengthen their armies by unearthing buried Zoids on Earth. The story follows Leo Conrad and Buzz Cunningham, second-generation Earth citizens who cross paths with Sally Land, a girl escaping the Empire's grasp.