Junior high school student Misaki Nanao lives in a town where the people around her are gradually losing their minds. Strange love letters, teddy bear abuse, and neighbors muttering nonsense are daily occurrences. One day, a peculiar demon-like creature appears in her home and claims that Misaki herself might be the crazy one. To prove she is the last sane person, Misaki begins recounting her terrifying personal experiences to the demon, whom she names Beherin. Beherin tells her that if she can convince him the world is mad, his growing magical power could help restore sanity. Yet as Misaki shares more stories, she is pulled deeper into the escalating insanity, horrific incidents, and a vast conspiracy.

The setting is a seemingly ordinary Japanese town that has become a hotbed of bizarre behavior. Students harm teddy bears for stress relief, teachers wear plague masks, a cult of people chanting “mimimi” roams the streets, and urban legends like the “Biku no Namakubi” (The Monk’s Severed Head) bring death to those who speak of them. Misaki’s only ally is Beherin, a rotund, three-eyed demon who loves ghost stories but is unreliable in a crisis. Her best friend, Gotō Michiru (Got-chan), is a kind girl who becomes entangled in the Biku no Namakubi mystery and is eventually murdered to silence her knowledge.

The central narrative arc revolves around the event known as “613,” which occurred on June 13 two years prior. On that day, former prime minister Mido Misaki gave a bizarre speech claiming that since 1875, all human minds had been partially replaced by the consciousness of higher beings, and that strange urges were instructions from those beings. Though a complete fabrication, five percent of the population believed it, spawning chaos, cults, and a wave of suicides and murders. Misaki herself was among the believers: as a child, she started a “613 Club” and inadvertently spread the “mimimi” mantra, which later became the Mimimi cult.

As Misaki investigates, she uncovers painful truths about her own past. She had forgotten she had an older sister, Minato, who hanged herself under the pressure of caring for Misaki and their brother. She also had a brother, Nagisa, who she blocked from memory after he failed to save her from a kidnapping. Beherin is revealed to be Nagisa, who has been protecting Misaki all along while playing along with her delusion. With Nagisa’s help, Misaki steals Mido’s will tape from the Mimimi cult. The tape contains an absurd promise of resurrection, which, when broadcast, shatters the believers’ faith and gradually restores sanity to the town. In the end, the madness lifts, Misaki reconnects with her brother, and the town returns to normal.