Koyomi Araragi notices something is terribly wrong while studying for his college entrance exams. Looking into a mirror, he sees no reflection, a definitive sign of a true vampire. After everything he has been through, his remaining vampiric abilities inherited from Shinobu Oshino have begun to consume his humanity. Seeking answers, he turns to the immortal oddity specialist Yozuru Kagenui and her shikigami, the expressionless doll-like tsukumogami Yotsugi Ononoki. The diagnosis is harsh but simple: Kagenui gives him two choices. He must either completely abstain from using his vampiric powers or continue down the path of becoming a full-fledged vampire, losing his humanity forever.

The story takes place in the familiar setting of the Monogatari series, a Japanese city where supernatural oddities manifest from human emotional struggles. However, the conflict quickly escalates beyond a personal dilemma. An antagonist specialist named Tadatsuru Teori kidnaps Araragi's younger sisters, Karen and Tsukihi, along with his kouhai Suruga Kanbaru, to force his hand. The narrative arc follows Araragi and Ononoki as they confront Teori, testing the protagonist's resolve and forcing him to face the direct consequences of his long history of meddling with the supernatural. As Araragi is forced to choose between risking his own soul or saving his loved ones, the story explores the theme of consequence, showing how his past actions to save others are now accelerating his own monstrous transformation. The climax reveals a deeper conspiracy, suggesting that the entire conflict was orchestrated to isolate him, leading to a resolution where Araragi must rely on his allies and accept the limits of his power, even as he privately acknowledges he would break his promise to abstain if his family were ever in danger again.