Suddenly appears in the world of fans of biological " ARU empty ". Because of their arrival, the human face extinction crisis. To break a strong force " the emptiness", only the root of " the Holy Grail of human ". So people will be in the world can take out and use the "Holy Grail " combat capability are together, established the air expeditionary army. As the soul. "Holy Grail " as weapons, was selected with the " Aru empty " fighting men, that is " Gorat knights ".