By the close of the 21st century, the discovery of a new power source dubbed "DG energy" (an abbreviation for "divine grace energy") has transformed Earth's economy and enabled human colonization of space. Yet, upon the initial expedition's arrival at Pluto, they face an assault from a mysterious, colossal life-form identified as "Nergal." In response, the Earth Alliance Army establishes an "AG Unit" utilizing ancient robotic weapons called the "G's Frame," which were unearthed from archaeological sites globally. Only girls possessing specific traits, referred to as "Ancient Girls," are capable of piloting these machines against the Nergal. Nevertheless, the inaugural AG Unit endures significant losses during the Jupiter Trojan campaign, and the pilot of Unit 04, Minamiya Reiu, is reported missing in action... Some years later, Minamiya Reika, Reiu's younger sister, joins the AG Unit to locate her sibling and defend Earth against the Nergal. What follows is a narrative centered on friendship, ancient robotic weaponry, and lethal confrontations with the Nergal.