Onihei tells the story of a man who was feared and called an "oni" by extremely brutal thieves in the late Edo period. This man was the "oni no Heizo" and the fire thief Kaikata Hasegawa Heizo. The fire thief Kaikata was a type of special police. They were not only responsible for suppressing crimes in and outside Edo City, but also able to go to other countries to catch criminals. It was a highly mobile organization. However, Heizo was not a stubborn man who was only loyal to his duties. Sometimes he would show mercy to criminals. What Heizo would never allow was inhumanity. The "Three Rules of Thieves" - do not kill people, do not rape women, and do not steal from stores that are about to go bankrupt - once these three rules are violated, those who betray humanity will never be forgiven.