In the wake of the 21st century's closure, the unearthing of 'DG energy,' a miraculous power source, reshapes Earth's fortunes and propels humanity into the cosmos. Yet, on reaching Pluto, an enigmatic colossal entity dubbed 'Nergal' launches a devastating assault on the inaugural mission. Responding with resolve, the Earth Alliance Army unveils the 'AG Unit,' fashioned from the ancient robotic arsenal of the 'G's Frame,' unearthed across global archaeological sites. Steering these mechs are gifted young women, dubbed 'Ancient Girls,' their mission to thwart the Nergal threat. Amid the grim toll of the Jupiter Trojan conflict, the disappearance of Unit 04's pilot, Reiu Minamiya, casts a shadow. Years later, Reiu's sibling, Reika Minamiya, joins the AG Unit to unravel the mystery of her sister's vanishing and shield Earth from the Nergal menace. A saga unfolds, weaving bonds of camaraderie, the legacy of ancient machines, and perilous clashes against the Nergal foe.