This is a very complex and tragic story. It is set in the pre-apocalypse of 1999. Long ago an angel, Alexeil leads the demons to challenge the angels of heaven lead by her twin brother, Raziel. However, Alexeil was defeated but with her remaining power she seals her brother within Earth so that he will be imprisoned forever. For this Alexeil is punished by infinite reincarnations which each dying a tragic death. Then the story starts with Setsuna Modou, a young disorientated boy, who is Alexeil's reincarnation. WIthin him lies a hidden desire to love his younger sister, Sarah. Sarah too loves him. But their love are doomed from the beginning.With Demons reincarnating into bodies of their friends, their sole goal is to awaken Alexeil to lead them again to challenge the heavens. However, Setsuna don't care much except for his love of his sister but fate is cruel when Razeil is freed from his seal. He then plan to kill Sarah. He feels that it's her fault for taking his beloved sister away from him. This time, Setsuna has no choice but to awaken the Alexeil inside him to protect the one he loves.