One day, at the base of Mt. Fuji, a "castle" appeared with Youkai pouring out of it. To curb the labour shortage caused by a declining population, the government decided to integrate the Youkai into society; however, this led to rising conflicts between them and humans. 
 
In response, the JSDF, with aims to expand their influence in public security, secretly forms a Youkai penal tactical response force, "Tachikawa Task Force", to execute "tactical solutions" for the various issues born from this new society.
 
(Source: Kadokawa, translated)
 
Note: Includes a bonus chapter.