The story of Nekomonogatari (Black) unfolds during Golden Week, a string of holidays starting at the end of April, and serves as a prequel to the events of Bakemonogatari while taking place after the vampire encounter chronicled in Kizumonogatari. The protagonist is Koyomi Araragi, a high school student who has recently been restored to being mostly human after a disastrous spring break left him a half-vampire. He finds himself preoccupied with confusing feelings he has developed for the perfect class president, Tsubasa Hanekawa, the girl to whom he owes his life. Unsure if his emotions are genuine love or mere frustration, he seeks advice from his younger sisters in a scene filled with their signature silly banter. The narrative quickly pivots from this comedic uncertainty to a darker mystery when Araragi encounters Hanekawa on the street and notices a bandage on her cheek. She confesses that her foster father struck her during a family argument, a symptom of the deep and hidden stress in her home life where she sleeps in the hallway and lacks a room of her own.

The central conflict ignites when Hanekawa and Araragi come across the corpse of a white cat with no tail that has been hit by a car. After they bury the animal, Hanekawa begins to act strangely. Araragi consults his mentor, the eccentric specialist Meme Oshino, who resides in an abandoned cram school alongside the lethargic vampire remnant Shinobu Oshino. Meme reveals that the creature they buried was not a real cat but an oddity known as a Sawari Neko, or cursed cat, which thrives on and amplifies stress. He warns that the apparition is the perfect vessel for Hanekawa in her fragile emotional state. The situation escalates rapidly when Araragi discovers that Hanekawa has been fully possessed by the oddity, transforming into a violent alternate persona often called Black Hanekawa. In this new form, she has drained the energy from her parents, sending them to the hospital, and begins rampaging through the town at night, indiscriminately attacking people to relieve the immense stress she can no longer contain.

The narrative arc, titled Tsubasa Family, follows Araragi as he grapples with a terrible realization: Black Hanekawa is not merely a victim of possession but an embodiment of her own suppressed will and desire to escape her painful reality. She is too intelligent and the oddity is too integrated for conventional exorcism. Over the course of five nights, Araragi is forced into a brutal cycle of confronting the rampaging cat, suffering devastating injuries, and being healed by Shinobu. The resolution comes through a desperate gambit where Araragi, using the legendary oddity-slaying sword Kokorowatari, sacrifices himself to inflict a fatal wound on Black Hanekawa. In the final moment, Shinobu, acting on his behalf, drains the cat of its energy, effectively severing the possession. The Golden Week nightmare concludes with Hanekawa regaining consciousness, having lost all memory of her rampage. Araragi is left with the burden of knowing the truth about her hidden suffering and the hypocrisy of his own feelings, recognizing that he could not truly save her but only suppress the problem, sealing the nine days of terror away as a dark secret. The story closes the first season of the Monogatari series, laying essential groundwork for Hanekawa's later character development and the return of the cat oddity in Nekomonogatari (White).