2014, fifteen years after a war between Earth and Mars. Desiring peace, Princess Asseylum of Mars holds a goodwill parade on Earth. Slaine a boy originally from Earth prays for her safety Meanwhile, on Earth, high school student Inaho and his friends receive pilot training in combat robots called Kataphraktos.

The Landing Castles of the Mars Knights that have landed on Earth launched an attack using Kataphraktos with the aim of invading the planet. The United Forces of Earth launch a counteroffensive. As the fierce battle rages on, Inaho encounters a mysterious girl.

To complete his plan, the Mars Knight relentlessly pursues Inaho and the others. Inaho is suspicious about the manner in which that knight fights. With backing from Marito’s group, Inaho begins a counteroffensive. Meanwhile, Slaine, unaware of the truth, has been ordered to provide support for the Mars Knight and is out in the field with him.

Slain has misgivings about the war. However, he has no allies around him and no way of telling who the enemy is. Meanwhile, Inaho is entrusted with a secret request by Seylum as Rayet, who is hiding her own true identity, is on edge.

Inaho and the others are hard pressed in their fight against the Martian Kataphraktos. But then a mysterious fighter craft appears and joins the battle. Elsewhere, Darzana and her crew hide the damaged assault ship in a dock, and there, they discover a combat mecha of unknown origin.

While Inaho and the others are carrying out their plan, they encounter Count Saazbaum’s Kataphraktos. Slaine, who had been trying to catch up to Princess Asseylum, finds himself in an unexpected situation. The fierce battle on the snowy field builds in intensity, and then—

The Mars Knights have assembled at Marineros Island. Slaine tries to take part in the battle, but he is looked down upon by his erstwhile fellow knights because he was born on Earth. Elsewhere, the crew of the Deucalion prepares for battle at Trident Base. Learning of the existence of Slaine’s swift Tharsis, they are wary of its capabilities.

The UFE is overwhelmed by the Mars Knights’ advance. Inaho and the Deucalion take part in the fighting, but they find themselves hard pressed by an unexpected attack. Elsewhere, Slaine unburdens himself to Eddelrittuo. A Mars Knight who requested an audience with Princess Asseylum did so with an ulterior motive.

The war has come to one final battle. Having reached an understanding, Slaine resigns himself to his fate. Elsewhere, Inaho answers the call of his own destiny. There is someone who sheds tears at the sight of the battlefield where all too many lights flare, then die out. A solitary girl saw the final moments of their battle.