In the distant future, people build their homes and raise domestic animals in subterranean caverns called "pits." This way of life has persisted for hundreds of years, and yet, the people are still powerless against the earthquakes and cave-ins which occasionally devastate their villages. Jeeha is one such village... One day, while digging tunnels to expand Jeeha's edges, a boy named Simon happens to find a small, shiny drill. Meanwhile, Kamina - Simon's soon-to-be "bro" - insists that there is another land above the village and tries to break through the ceiling in order to leave the cavern. He fails; however, an earthquake suddenly occurs, the ceiling collapses, and a gigantic robot - a "Ganmen" - crashes to the cave floor! Kamina is now certain that there is a world above their tiny pit-village. Encouraged, he begins to fight recklessly against the robot, until he encounters another person who has arrived from the land above. She is a beautiful girl by the name of Yoko who has come from a neighboring village with a rifle in hand. She has been hunting the robot, but her shots only seem to irritate it. Desperately trying to escape its attacks, Simon brings Kamina and Yoko to what he had previously found: A mysterious robot with a head for a body. And so begins an adventure that takes "epic" to a whole new level...