"Our Last Crusade or the Rise of a New World" is set in a world divided by an ongoing war between two powerful factions: the scientifically advanced Empire and the mystical Sovereignty. The Empire relies on cutting-edge technology and military might, while the Sovereignty harnesses the power of astral energy, granting its inhabitants supernatural abilities. The conflict between these two nations has persisted for years, with no end in sight.

The narrative centers on two key figures from opposing sides: Iska, a skilled swordsman and former soldier of the Empire, and Aliceliese "Alice" Lou Nebulis IX, the Sovereignty's Ice Calamity Witch and a princess of the Nebulis family. Iska, once a promising soldier, becomes disillusioned with the Empire after a traumatic event and is imprisoned for defying orders. Alice, on the other hand, is a prodigious astral mage tasked with leading the Sovereignty's forces against the Empire. Despite their roles as enemies, their paths cross during a mission, leading to a fateful encounter that challenges their loyalties and beliefs.

As the story progresses, Iska and Alice form an unlikely alliance, driven by a shared desire to end the war and uncover the truth behind the conflict. Their journey reveals deeper layers of the world's history, including the origins of astral energy and the true motives of both factions. Along the way, they encounter allies and adversaries, including Rin, a loyal and resourceful soldier from the Empire, and Mismis, a cheerful and determined captain who supports Iska. On the Sovereignty's side, characters like Jhin, a stoic and powerful mage, and Nene, a mischievous yet skilled fighter, play significant roles in shaping the narrative.

The series explores themes of loyalty, identity, and the cost of war, as Iska and Alice navigate their growing bond while grappling with the expectations placed upon them by their respective nations. Their relationship becomes a focal point, symbolizing the possibility of reconciliation between the Empire and the Sovereignty. However, external forces and internal conflicts continually test their resolve, as both factions harbor secrets that could either unite or further divide the world.

The plot is driven by a mix of action, political intrigue, and character development, with battles showcasing the clash between technology and magic. The narrative balances moments of tension with quieter, introspective scenes, highlighting the personal struggles of the characters. As Iska and Alice work toward their shared goal, they uncover hidden truths that challenge their understanding of the war and their roles within it, setting the stage for a larger confrontation that could determine the fate of their world.