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. Suddenly caught up in an international dispute, Yuinshiel must preserve the security of her country while also protecting Rena from those who wish to abuse her powers. However, by fighting for her sister, Yuinshiel risks triggering the same calamity that once destroyed Rimgarde. Amid such a precarious situation, the two girls push their bond to its limits as they fight not only for themselves but for the world's safety as well.

When Subaru Natsuki leaves the convenience store, the last thing he expects is to be wrenched from his everyday life and dropped into a fantasy world. Things are not looking good for the bewildered teenager; however, not long after his arrival, he is attacked by some thugs. Armed with only a bag of groceries and a now useless cell phone, he is quickly beaten to a pulp. Fortunately, a mysterious beauty named Satella, in hot pursuit after the one who stole her insignia, happens upon Subaru and saves him. In order to thank the honest and kindhearted girl, Subaru offers to help in her search, and later that night, he even finds the whereabouts of that which she seeks. But unbeknownst to them, a much darker force stalks the pair from the shadows, and just minutes after locating the insignia, Subaru and Satella are brutally murdered. However, Subaru immediately reawakens to a familiar scene—confronted by the same group of thugs, meeting Satella all over again—the enigma deepens as history inexplicably repeats itself.

Subaru Natsuki finally gets to take a breather, but he does not waste any time as he prepares for a date with his beloved Emilia. He scouts the nearby village for the right dating spot, and with the help of the village children, he finds a wonderful location. With that, he is well prepared for his date! Unfortunately for Subaru, cold weather suddenly sweeps across Roswaal's mansion on his important day, leaving him with no choice but to postpone the date. Overnight, it becomes even colder and unbearable. Subaru must get to the bottom of this because, at this rate, his date will be the least of his worries.

Covered in ice and snow, Elior Forest is the home to dangerous magical beasts and 50 elves frozen in ice. One day, the great spirit Puck helps a young girl break out of her ice prison. Her name is Emilia, a half-elf born with silver hair, long ears, and amethyst eyes—features that resemble the evil Witch who destroyed half the world long ago. Shunned by society because of her appearance, Emilia dwells in the forest with Puck as her sole companion and family. Burdened with a sin of destruction she does not remember committing, she spends her days trying to find a way to help her frozen kin. But when the great spirit Melakuera, the Arbitrator of the world, finds Emilia, her right to stay alive is brought into question. Will the bonds of ice she formed with Puck prove to be the warm thread that defies fate?