Trained to combat demons, monsters, and other creatures in the post-apocalyptic 21st century, Kakugo and Harara are siblings equipped with superior fighting techniques and specialized skills. In addition, they both wear Zero armor, suits crafted from the souls of deceased warriors. However, when Harara falls victim to the darkness that lurks within her Zero armor, Kakugo becomes the sole human capable of defeating his own sibling, armed with his unique skillset and equipment.