In the kingdom of Rimgarde, two mysterious giant robots begin a duel that erases nearly all human life before they disappear. Twelve years later, the young empress of Enastoria, Yuinshiel Asteria, works tirelessly to maintain both her authority and the support of her citizens. But the peace of Enastoria is threatened when survivors of Rimgarde arrive. Armed with giant robots called Regalia Gear, they look for Yuinshiel's adopted sister, Rena. The latter is forced to reveal herself as not just a Regalia Gear wielder, but a real "Regalia"—a human capable of transforming into a giant robot—and the culprit behind the fall of Rimgarde.