The discovery of a new energy source known as "DG energy" (short for "divine grace energy") around the end of the twenty-first century changed Earth's economy and allowed humans to colonize space. When the first mission reaches Pluto, however, they are attacked by "Nergal," an unknown, enormous life-form. The Earth Alliance Army responds by forming a "AG Unit" that employs ancient robot weaponry known as the "G's Frame" that have been unearthed from archaeological locations all across the planet. These robots may be piloted by girls with extraordinary abilities known as "Ancient Girls." The first AG Unit, however, suffers heavy losses in the Jupiter Trojan campaign, and Unit 04's pilot, Minamiya Rei...