From before the dawn of time people ("Crimsons") have been controlled by a deadly foe ("Clients") who think themselves gods. Now, at the turn of the twentieth century, the Crimsons still live in fear of the Clients. Powerful individuals protect themselves by making sacrificial offerings of live human beings.
Dark Hunter, Shion, is half-Crimson, half-Client - in other words a vampire. With his loyal sidekick, Rain, he wages an endless war against the Clients. When Shion meets Helen--a young girl whose family were butchered by the Clients--he makes a blood oath and takes Helen as one of his followers.