There are worlds more than our own. This was shown on the day one merged with ours. Himura Ketsueki, an apathetic, unsociable boy with a curiousity for killing. Theresa Cassius, daughter of a noble Demon family, violent and ruthless on the outside, her inner self hidden underneath a facade. The journey begins as the world is turned upside down, a new era starts, and power is given to those who yearn for it.