In the late 21st century, humanity's access to space and economy underwent a significant transformation after discovering a new energy source called "DG energy" or "divine grace energy". The newfound energy led to the colonization of space; however, the first space expedition to Pluto is met with a hostile attack by a colossal life-form called "Nergal." To fight back, the Earth Alliance Army invents an "AG Unit." The AG Unit consists of the "G's Frame," antique robotic weapons from various archaeological sites, and "Ancient Girls" -- girls with unique abilities to pilot these robots to combat the Nergal. Unfortunately, Unit 04's pilot, Minamiya Reiu, becomes MIA after the disastrous Jupiter Trojan campaign, resulting in heavy casualties for the AG Unit. Years pass, and Reiu's younger sister Reika joins the AG Unit to locate her missing sister while defending the Earth from the wrath of the Nergal. A fascinating story of friendship, ancient robot weapons, and deadly fights against the Nergal follows.