The distant future. After reaching its peak, Earth's population fell into decline. AI was developed to help preserve the species and manage the ecosystem; a grand experiment in human diversity. Through genetic engineering, a diverse array of peoples, including beastfolk, magical beings, and more, was created to live in a diverse city separated into walled "clusters". Each cluster developed a unique culture, befitting of the people that lived there. Their lives, and the system, is continually maintained by the AI to ensure survival.

The OVA will be based on the Narumi Seo character map story event that was featured in the smartphone game in April 2018. Seo, who has a unique memory disorder, will appear alongside other heroes of the Seo Research Laboratory in the OVA.

Nearly twenty years into the 21st century, people have forgotten the existence of youkai. When a number of unexplainable phenomena plague adults of the human world with confusion and chaos, thirteen-year-old Mana writes a letter to the Youkai Post in search of answers, only to be greeted by Gegege no Kitarou...