Takashi Natsume has grown accustomed to his encounters with youkai through the Book of Friends, which contains the names of youkai whom his grandmother, Reiko Natsume, has sealed in contracts. These encounters allow Natsume to better understand the youkai, Reiko, and himself. 

The Book of Friends is a powerful tool that can be used to control youkai; it is sought after by both youkai and exorcists alike. Natsume just wants to live out his daily life in peace but is constantly disrupted by these experiences. If he is to end this torment, Natsume must explore more about the book and the world of exorcism, as well as begin to open his heart to those who can help him.

[Written by MAL Rewrite]

In the town of Kurouzu-cho, Kirie Goshima lives a fairly normal life with her family. As she walks to the train station one day to meet her boyfriend, Shuuichi Saito, she sees his father staring at a snail shell in an alley. Thinking nothing of it, she mentions the incident to Shuuichi, who says that his father has been acting weird lately. Shuuichi reveals his rising desire to leave the town with Kirie, saying that the town is infected with spirals. But his father's obsession with the shape soon proves deadly, beginning a chain of horrific and unexplainable events that causes the residents of Kurouzu-cho to spiral into madness.

Koyomi Araragi spent his last year of high school helping girls in his town resolve various supernatural afflictions. But now Araragi has departed for university, leaving his friends to fend for themselves against new problems and curses that plague them. Yotsugi Ononoki, once a human corpse and now a living doll, takes residence in Araragi's home, keeping watch over his sister Tsukihi, a girl harboring a mystical secret of her own. As part of her duties, Yotsugi fills Araragi's vacated role as occult expert, assisting others in town with their issues. One of these girls, middle school student Nadeko Sengoku, slowly recovers from her own recent brushes with the paranormal. She avoids returning to school, instead spending time alone in her room and pursuing her dream of becoming a professional manga artist. In order to speed up Nadeko's quest for mastery of her craft, Yotsugi convinces her to create four copies of herself, each representing a distinct aspect of Nadeko's personality. However, the clones refuse to help Nadeko, instead escaping into the town and creating a chaotic mess. Now forced to grapple with her own fractured sense of identity, Nadeko sets out to capture them and resolve her inner conflict.

Original video anime episode of Watashi no Shiawase na Kekkon.