An empress discovers she's a mecha pilot the moment her world stops being theoretical and starts being real. The show burns its entire runtime on one question: what does it cost to protect someone when the only weapon you have demands you become something else? Yui's love for her sister is the engine of every fight, not the backdrop, it's why each battle matters, why each transformation feels like a choice instead of a spectacle.

Twelve years ago, a mysterious clash of giant robots in the kingdom of Rimgarde wiped out almost all of its people. Now Yuinshiel Asteria, known as Yui, is the young empress of Enastoria, working hard to keep her authority and her subjects' support while living alongside her adopted sister Rena. That calm breaks when survivors of Rimgarde arrive with giant robots called Regalia Gear, searching for Rena. Pulled into an international dispute, Yui must keep her country safe while shielding her sister from anyone who would exploit her, in a mecha and sci-fi story built around the bond between the two girls.