In the declining years of the 21st century, the global economy underwent a transformation thanks to the discovery of DG energy, also called divine grace energy, which enabled humanity to expand into outer space. However, this advancement faced a threat when the first expedition to Pluto was suddenly attacked by Nergal, a massive and enigmatic life-form. To confront this danger, the Earth Alliance Army deployed AG Units, ancient mecha known as G's Frame, uncovered at various archaeological sites around the world. These units can only be piloted by special individuals referred to as Ancient Girls. During the Jupiter Trojan campaign, the AG Unit suffered heavy losses, including the disappearance of Pilot Minamiya Reiu of Unit 04 under unknown circumstances. Years later, her younger sister, Minamiya Reika, enlists in the AG Unit with the twin goals of finding her missing sibling and defending Earth from the Nergal threat. The narrative intertwines themes of friendship, ancient machinery, and fierce battles against a supernatural foe.
