Diego no Kyojin

It is an era of change that was brought about by the discovery of the New World. A young man named Diego, who lived in the maritime nation of Spinea, was a researcher of the art of Onmyojutsu which was considered illegal. Due to that, he was attacked by a mysterious young woman who doubted the source of his powers, and even the Holy Church started to take an interest in him. Diego, who had no other options, decided to board a ship that was headed on a research expedition to the New World along with his long-time friend Alvaro.

The New World had a Tree of Gold that towered over the surroundings, and the forests were protected by colossi. Waiting for the two of them there was the girl, Rera, who was confined in a prison of water. Diego was ordered by Captain Joaquin of the expeditionary force to use his Onmyojutsu and find a way to get her out. However when Rera awakens, the young lady who had attacked him in Spinea, Rozen, makes an appearance. She is the rider of the colossus "Tankamui."

Is Diego going to get caught up in the great battle between the maritime nation and the natives—?