When a cult organization, the Shiromorishuu, resurfaces and sends a monster to Tokyo, ASY, a peace-keeping organization in Japan, retaliates with the mechanized Gousen. ASY's Gousen annihilates the creature after driving it into open ocean. Although successful, a surprise attack follows. A girl, floating in midair and glowing with radiance, strikes with a beam of light. This girl is "Rouran." She knows naught her identity, her origins, nor why whenever she senses the Gousen or Kyoushi, she transforms into an avenging angel. As Rouran discovers the truth of her mysterious existence, Tetsuya kindly watches over her in Tokyo Arcadia, a settlement of expatriates and dissidents.