"Natsume Yūjin-Chō: Ishi Okoshi to Ayashiki Raihōsha" is a two-part original video animation (OVA) that expands on the world of the "Natsume Yūjin-Chō" series. The story centers on Takashi Natsume, a young man who possesses the ability to see yokai, spirits invisible to most humans. This ability stems from his grandmother Reiko, who also had the power to see yokai and left behind a book called the "Book of Friends," containing the names of yokai she had defeated in challenges. The yokai whose names are written in the book are bound to it, and Natsume seeks to return their names to free them.

In this OVA, Natsume encounters a yokai named Ishi Okoshi, a small, rock-like spirit tasked with waking up a sleeping deity in the mountains. Ishi Okoshi seeks Natsume's help to complete this duty, as the deity's prolonged slumber has caused disruptions in the natural balance of the area. However, the task is complicated by the arrival of mysterious and hostile yokai known as the Ayashiki Raihōsha, who aim to prevent the deity from awakening. These yokai are drawn to the Book of Friends, further endangering Natsume and his companions.

Throughout the story, Natsume is accompanied by his loyal yokai guardian, Madara, who often takes the form of a rotund, cat-like creature and goes by the name Nyanko-sensei. Nyanko-sensei provides both comic relief and crucial assistance in Natsume's encounters with yokai. The narrative explores themes of responsibility, the coexistence of humans and yokai, and the importance of understanding and compassion. Natsume's interactions with Ishi Okoshi and the Ayashiki Raihōsha highlight his growing resolve to protect both the human and yokai worlds, even as he grapples with the challenges of his unique abilities.

The OVA maintains the series' signature blend of supernatural intrigue, emotional depth, and moments of lightheartedness, offering a self-contained story that enriches the overarching narrative of "Natsume Yūjin-Chō." It provides further insight into Natsume's character and his evolving relationships with the yokai he encounters, while also introducing new elements that expand the series' lore.