The Medicine Seller is a deadly and mysterious master of the occult who travels across feudal Japan in search of malevolent spirits called mononoke to slay. When he locates one of these spirits, he cannot simply kill it; he must first learn its Form, its Truth, and its Reason in order to wield the mighty Exorcism Sword and fight against it. He must begin his strange exorcisms with intense psychological analysis and careful investigative work—an extremely dangerous step, as he must first confront and learn about the mononoke before he even has the means to defeat it.

Musashino, Tokyo. An elementary schooler repeatedly attacks people in the streets. Known only to the public as "Lil Slugger", none of the victims can recall the young boy's face and only three distinct details are left in their memories: golden inline skates, a baseball cap, and the weapon: a bent golden baseball bat. Detectives Ikari and Maniwa set out to track down the perpetrator and put an end to his crimes.