Koyomi Araragi receives an unexpected request from Iezumi, an associate professor of linguistics, who asks him to save her child from her own abuse. Iezumi has reportedly been told by Sodachi Oikura that Koyomi is an expert on child abuse, a reputation stemming from his past involvement with Oikura's own troubled family situation. As Koyomi rushes in his beloved car to the professor's apartment, the scene of the alleged abuse, he is suddenly confronted by the eerie, doll-like apparition Yotsugi Ononoki. With no clear explanation for why she has chosen to accompany him, the pair proceed together, only to bear witness to an astonishing and disturbing sight at their destination.

Amarimonogatari is the twenty-fifth book in Nisio Isin's overall Monogatari series and the third installment in the Monster Season arc, following Yoimonogatari. The story is set after the events of the Final Season, with Koyomi now navigating young adulthood while still tethered to the world of supernatural oddities. The novel focuses heavily on Yotsugi Ononoki, the shikigami tsukumogami created by the eccentric specialist Yozuru Kagenui. Yotsugi, who often speaks in a stilted, emotionless manner and punctuates her sentences with a signature pose, has been a recurring presence in Koyomi's life since his high school days. The title derives from the amari reading of the kanji in Yotsugi's name, which carries connotations of surplus, remainder, and the self.

The narrative unfolds across two main arcs. In Yotsugi Body, also known as Yotsugi Buddy, Koyomi and Yotsugi investigate the disturbing situation at Iezumi's apartment, confronting a dark facet of human behavior that blurs the line between personal failing and supernatural affliction. The second arc, Yotsugi Shadow, continues to explore the bond between Koyomi and the oddity girl, delving into the nature of her existence as a leftover or remainder of past events and the strange purpose she serves in Koyomi's life. Throughout both stories, the novel examines themes of abuse, responsibility, and what it means to be an outsider looking in on a family's suffering. The only human authority figure involved is the linguistics professor Iezumi, whose gender is never specified, and her child, while new oddity phenomena also surface as Koyomi and Yotsugi uncover the full truth behind the desperate plea for help.