Araragi finds himself chained up in Oshino’s old hideout by Senjougahara, having been kidnapped by her. The scene then shifts to a hot summer’s day, where he ponders on what to do with his unstable vampirism and whether he should tell his two sisters, Karen and Tsukihi about oddities. Deciding to meet up with Sengoku, he heads of to her house but spots Hachikuji Mayoi and stops to chat (molest) to her.

Araragi turns up at Sengoku’s house, and tries to not get turned on by her lack of clothes and seductive advances at a time when her parents are conveniently out. After he leaves, he meets his sister Karen on the way to Kanbaru’s house, where he realizes that the Fire Sisters are up to something suspicious but decides not to interfere. When he catches Kanbaru completely naked and starts questioning how much of a pervert she is, Kanbaru jumps him and starts taking off Araragi’s clothes, and he promises to never molest Hachikuji again.

Araragi finishes up cleaning at Kanbaru’s house, and proceeds to head home on his bike. Right outside of Kanbaru’s he sees a strange, ill-omened man, named Kaiki Deishuu who he instantly relates to having a similar aura to Oshino. When he meets Senjougahara on the way home and mentions Kaiki, she chains him up and tells him that Kaiki was one of the conmen that took away her family’s money. When Araragi gets a text from his sister asking for help, he easily breaks free of his chains and Senjougahara lets him go after talking to Hanekawa.

Araragi tries to get Karen and Tsukihi to tell him about the incident, however they refuse and Tsubasa has to explain to him that while investigating Sengoku’s curse, Karen met up with Kaiki Deishuu and had something done to her. Later on, while in the bath, Shinobu suddenly comes out of Araragi’s shadow again and tells him that she recognizes that Karen has an oddity Oshino once told her about – the Wreathe-Fire Bee. The two end up reconciling, with Araragi realizing that if he has the lifespan of a vampire, Shinobu will be the only one left beside him after hundreds of years.

Tsukihi and Tsubasa tell Araragi about the meeting between Karen and Kaiki Deishuu, who infects Karen with the Wreathe-Fire Bee after a monologue about his beliefs on the importance of money. After Tsubasa leaves, Araragi helps to freshen up a still spirited Karen, who doesn’t want to pass on the fever to him. When Araragi asks Shinobu about it, he realizes that the curse can actually be passed onto him to cure Karen.

Koyomi only managed to drain half of the poison in Karen's body, and although she was not completely healed, her fever has become slightly diminished. On his way to Hitagi's house, Koyomi stumbles on Mayoi again and confides to her about his sister's situation. Upon arriving at Hitagi's house he finds out that Hitagi managed to arrange a meeting with Kaiki and is preparing herself to enact revenge on him. Koyomi explains to her about Kaiki's encounter with Karen and convinces her to not confront him without his help. Back at home, Koyomi learns from Tsukihi that Karen is missing, and worried that she might had left to face Kaiki once more, he asks Shinobu's help to find her.

Thanks to Shinobu, Koyomi manages to find Karen and after a serious fight between them, he convinces her to let him deal with Kaiki by himself. Later that day, he and Hitagi come face to face to Kaiki, but they hear from him, much to their surprise, that he is already leaving the city and Karen should recover in a few days. After letting off all anguish she harbored on him all those time, Hitagi lets Kaiki leave unharmed, and claiming she can finally move on with her life, she asks Koyomi to spend the night with her. In the next day, Koyomi returns home to find that Karen is already fully recovered, and both his sisters leave in a hurry, claiming that there is still work for them to do, due to all the damage Kaiki left behind among their classmates.

Koyomi is concentrated on his studies when he is interrupted by Karen who, much by his surprise, is wearing her sister Tsukihi's clothes, displaying an air more feminine than usual. Karen asks Koyomi to introduce her to Suruga knowing that she is friends with him, but he refuses without giving a reason, fearing that she would make a move on his sister. Karen then challenges his brother to a contest, making him promise to comply with her request if he loses. Koyomi then choses the contents of the challenge - she would let him brush her teeth for five minutes without resisting, which she accepts. Surprisingly, both start getting more and more aroused as Koyomi brushes Karen's teeth until Tsukihi barges in demanding an explanation. Despite fearing what punishment Tsukihi has in store for them after she leaves and Koyomi accepting defeat, Karen asks him to continue from where they stopped.

Koyomi sets for Suruga's house to introduce Karen to her as he promised. On their way they meet a woman introducing herself as Yozuru Kagenui, asking for directions. Koyomi asks for Tsubasa's help on his phone and learns from her that she is asking for the location of the same building Oshino was living with Shinobu. Before leaving, Yozuru asks Koyomi to give the same information to a certain girl when he meets her, and Karen states that despite Yozuru's strange behavior, she had the look and feel of being a fighter much stronger than her. After leaving Karen at Suruga's, Koyomi meets Mayoi upon returning home and they are approached by a girl introducing herself as Yotsugi Ononoki, asking for the same directions, just like Yozuru had predicted, and much to his surprise Koyomi finds that not only Yotsugi can see Mayoi, but both Yotsugi and Yozuru know about his and his friends' circumstances.

Shinobu asks Koyomi to take her to the donuts shop. Before leaving, he notices that Tsukihi's scar on her chest was healed, and after stripping her naked, finds that all other scars on her body disappeared too. At the shop, Koyomi asks Shinobu if she knew anything about Yotsugi or Yozuru, but much to his surprise, he finds Kaiki sitting on a nearby table and pays him up for information about them. Kaiki reveals that Yozuru and her familiar Yotsugi are supernatural hunters specialized in immortal beings. Wondering if he or Shinobu may be targeted by them, Koyomi returns home just to find both in front of it. When Tsukihi opens the door for them, Yotsugi instantly attacks her, tearing her body apart. A furious Koyomi launches himself at her, but is easily subdued by Yozuru, who shows him that surprisingly, Tsukihi's body is already restored. She also explains that the Tsukihi in front of him is not his true sister but a phoenix assuming her identity. Shinobu threatens to attack the duo, and Yozuru, despite knowing that the vampire has not enough power to stop her, decides to leave, promising to return in the following day to slay Tsukihi, killing both Koyomi and Shinobu too if they try to stop her again.

After learning from Shinobu that Tsukihi is actually a supernatural being that endlessly reincarnates as a human child after dying, Koyomi leaves Karen standing guard at their house's destroyed front door before setting to the place Oshino used to live to confront Yozuru and Yotsugi. To prepare for their clash, Shinobu draws some of Koyomi's blood to recover her true form and agrees to fight Yotsugi while her master deals with Yozuru. Yozuru reveals to Koyomi that she, Oshino and Kaiki were friends from college and it was Kaiki who told her about Tsukihi being a phoenix. After viciously attacking Koyomi, Yozuru learns that her partner was easily defeated by Shinobu, and just when she is about to confront the vampire, Koyomi rises again, claiming that he would never give up on his sister, no matter what kind of creature she is. Realizing his determination to protect Tsukihi and that he never fought with the intention to kill her, Yozuru decides to give up and leaves peacefully with her familiar. Koyomi returns home to check on Tsukihi and declares to her that he will soon introduce his girlfriend to his sisters. In the end, Koyomi is seen meeting Hitagi at a beach.