In a post‑apocalyptic Earth of the 21st century, monsters, mutants, and demons roam the wasteland, hunting human prey. Kakugo and Harara have been trained by their father in the ancient Zero fighting technique, a discipline designed to protect the last remnants of humanity and restore hope and peace to the world. With their superior combat skills, secret techniques, and a special living armor forged from the souls of ancient warriors, the siblings are dispatched across the devastated landscape to defend the weak from the relentless assaults of demons and mutants. However, despite their rigorous training, Harara falls to the darkness within, kills their father, and believes that Kakugo has also perished.