Once, a catastrophe known as the "Great Collapse" brought human civilization to a complete and utter end. The surface became a hell of toxic fog, where grotesque remnants of what once was roam. In the deepest level of a decayed, colossal military shelter, just the two of them are left in the world.

In the windowless underground shelter, an unidentifiable, deep rumbling echoes again today. In the corner of that room, on what was once a maintenance bench, the man sits slumped. Kai. A humanoid weapon created by humanity in its final days. Incredibly close to human, yet capable of annihilating an entire division alone. That "ultimate weapon" is now hugging his knees like a small child, repeating shallow, sharp breaths—hyu, hyu.