Renji Sato knows the taste of the canvas better than anyone. In Tokyo's amateur fighting circuit, he is known as "The Punching Bag": a young man with encyclopedic technical knowledge, but trapped in a mediocre body that refuses to obey.

On the verge of giving up on his dream, a night cleaning job at an ancient temple reveals a hidden door that shouldn't exist. It leads to a pocket dimension where an enigmatic Mage is imprisoned. The Mage offers Renji an offer he can't refuse: the power to become the champion he always yearned to be.

The price? Renji must serve as the Mage's warden in the human world, hunting spiritual creatures that hide in the shadows of Shinjuku and feed on the unsuspecting populace. Now, Renji lives a brutal double life: fighting for trophies under the spotlights and battling for survival in dark alleys.