In the 23rd century, humanity built a particle collider spanning continents in its quest to uncover the truth of the universe. But the moment it powered up and collided its particles, a crack split open in the sky. A mysterious energy enveloped the Earth, and terrifying alien beasts poured out of spatial rifts.
Thus began a war between humanity and the alien beasts that would last for decades. As increasingly powerful beasts descended, humanity was driven back at every turn.
At last, the leaders of every nation led the surviving humans in establishing the Blue Star Federation. They pooled all their resources to build a steel city more than a thousand meters tall, naming it “Fortress.” For the time being, it held back the alien beasts.
As time passed, the crack grew wider, and the arrival of even more terrifying beasts drew nearer. The human federation fell into despair.
Then a handsome young man burst into the federation’s command center, and a new era began...
