"Divine grace energy" or DG energy has transformed Earth's economy and opened up the possibility of space colonization in the late 21st century. However, during an expedition to Pluto, the team encounters an attack by a mysterious creature named "Nergal." In response to the Nergal threat, the Earth Alliance Army develops "AG Units" utilizing ancient robotic weapons known as "G's Frame." These machines can only be piloted by "Ancient Girls," young women possessing unique qualities. The initial AG Unit suffers significant losses during a Jupiter Trojan campaign, resulting in the disappearance of Unit 04's pilot, Minamiya Reiu. Years later, Reiu's sister, Reika, joins the AG Unit to search for her missing sibling and battle against the Nergal. It is a journey filled with friendship, ancient robot warfare, and a determination to go the distance against the Nergal.