In a world where the boundary between the known and unknown is blurred, a young nobleman is forced to close his home in Kyoto and move to the new imperial court in Fukuhara. However, his loyal servant stays behind to look after the house and finds it infested by one hundred yokai who refuse to leave. The nobleman prays to Buddha, and by dawn, the demons are thrown out of the house by the power of religion. But that is only the beginning of his journey. On his way to the house, the nobleman encounters the Hyakki Yagyo, a parade of demons, yokai, and other supernatural beings that flood the town streets at midnight. The monsters overwhelm his carriage, chase away his attendants, and set about destroying the vehicle. Ogres smash the carriage with their mallets, while demons set it on fire with golden fireballs.
With Buddha's help, the nobleman manages to escape into a field. Stopping to rest, he prays until the break of dawn. When the light comes, the demons scatter back into the darkness.
But as he travels further, he discovers that the Hyakki Yagyo is not just a single event but a thousand-plus-year-old Japanese folkloric tradition in which a series of demons parades-or explodes-into the ordinary human world. The procession is made up of various monsters such as skeletal soldiers riding a human-headed horse; a frog-like demon dominating a badger; furry-headed demons and naked demons.
The young nobleman soon realizes that he has been chosen to stop this chaos from spreading further. He must use all his wit and courage to face these supernatural beings and prevent them from causing destruction and havoc in the mortal world. He learns that chanting magical spells such as KA-TA-SHI-HA-YA, E-KA-SE-NI-KU-RI-NI, TA-ME-RU-SA-KE, TE-E-HI, A-SHI-E-HI, WA-RE-SHI-KO-NI-KE-RI will protect him from the demons.
As he delves deeper into this mysterious world, the young nobleman encounters a one-eyed, malformed goddess, Benzaiten, accompanied by an elephant, reptilian monsters, and the fierce Buddhist guardian Fudō Myōō holding a sword. He also learns that the Hyakki Yagyo is made up of three types of plots: the type of the nocturnal march across the streets of the capital; the haunted house type; and the type of the demonic egression in the mountains.
The young nobleman's journey is full of twists and turns as he battles various demons and yokai, including nurarihyon, otoroshi, and tsukumogami. These creatures test his strength and determination, but he perseveres, knowing that he is the only one who can stop the Hyakki Yagyo from causing chaos and destruction in the mortal world.
