A forest suddenly appeared in Shibuya. There was a mysterious phenomenon... people going to the city were suddenly transforming into trees! Meanwhile, Mana, a junior high school student, dubiously wrote a letter and put it in the Yōkai Mailbox. Soon after, Kitarō came with a "clip, clop." After hearing Mana's story, Kitarō and his father, Medama-Oyaji, were convinced that this was the work of a yōkai that should have been sealed hundreds of years ago. To find the cause, they traveled to the city's underground facility. The one that was awaiting them is...!?