When the secret occult society, Shiromorishuua, sends a huge monster to attack Tokyo, ASY, a peace-keeping organization in Japan, retaliates with a Gousen (metal fairy). 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 neither her identity, her origins, nor why whenever she senses the Gousen or Kyoushi (reanimated corpses) she transforms into an avenging angel. As Rouran worries about her mysterious existence, Tetsuya kindly watches over her in Tokyo Arcadia, a settlement of expatriates and dissidents. Who is this Rouran person? How will this story develop? Find the answers in Kikou Sennyo Rouran.

Dr. Masahiko Mikogami created the organization ASY, and it's five mechs called Gousens, to act as the new defense force for Japan after it's own military joined the UN. ASY immediately went after the Shiromorishu, a mysterious group that guided Japan spiritually for centuries, stating that the group had been controlling Japan instead of guiding it. Though their base has been destroyed and their successor, Tatsuya Takechi, is gone and possibly dead the Shiromorishu are ready to enact their vengeance on ASY... But a mysterious girl named Rouran will complicate matters further by bringing back the past between Mikogami and the Shiromorishu.—animenewsnetwork

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.