The angel Alexiel loved God, but she rebelled against Heaven when she saw how disgracefully the other angels were behaving. She was finally captured, and as punishment sent to Earth to live an endless series of tragic lives. She now inhabits the body of Setsuna Mudo, a troubled teen in love with his sister Sara. Setsuna's misery mirrors the chaos among the angels, and their combined passions threaten to destroy both Heaven and Earth.

Why is Setsuna so mixed up? Despite his attempts to be noble, he'll fight anyone anytime; he ignores all authority; and he harbors feelings for his sister that can only be described as "incestuous." Why is he such a mess? The reason may be found with two demons from the underworld and their enemy, an insane angel who is distributing an insidious computer game called Angel Santuary.