On the day of Yuto's 16th birthday, he and his childhood friend Rinko meet a mysterious katana-wielding girl on their way to school. The girl starts to lick Yuto face much to Rinko's shock, forcing her to drag him away. At school during a heated arguement on the rooftop, Yuto and Rinko are attacked by Taizo who is possessed by a demon spirit until the mysterious girl from earlier excorcises it and kills it. Later that evening, Yuto is met once again by the mysterious girl who introduces herself as the cat spirit Himari, and explains he is being targeted by ayakashi who are out to kill him and that she will protect him from them. However, Rinko appears unexpectedly and causes a ruckus after seeing them together. The following day, Himari transfers to Yuto and Rinko's school. The night before, Himari reveals that Yuto's family is one of the twelve Demon Slayer families that killed ayakashi for hundreds of years, and that one of her ancestors was spared by Yuto's ancestor that led to a vow for her kind to forever serve them. Refusing to accept this fact, Rinko challenges Himari to a sports battle for Yuto's custody, only for her to lose in all of them. After being told by Himari that she lacks the strength to protect Yuto, Rinko insults her by calling her a monster. After school, Yuto reconciles with Himari on the rooftoop, who asks if it is okay to have an ayakashi like her around him. Yuto, understanding that she is only concerned about his well-being like his grandparents were, allows her to perform her duty. The next morning, Rinko comes to apologize to Yuto only to see Himari in his bed to her discomfort and it turns into yet another ruckus. Meanwhile, the ayakashi plan their move to kill Yuto when a mizuchi named Shizuki appears, claiming she will kill him herself.

In an attempt to make Yuto remember his forgotten past, Himari decides to invite him to go to Noihara with her. Unfortunately, Rinko and Shizuku tag along with them on their own agendas to Himari's disappointment. Upon arriving at Yuto's grandparents' house, they are greeted by the Zashiki-warashi Kaya, who bears a grudge against Yuto for having Himari leave her by herself. Later on, after remembering his past with Himari through a walk in the forest, the Hinoenma Ageha appears for Yuto's life, and just when they are about to retreat to the house, a Daidarabotchi blocks their path. Back at the house, Rinko, Kaya, and Shizuku are attacked by an ippon-datara named Sasa. Back at the forest, Himari has a tough time fighting Ageha and the Daidarabotchi and orders Yuto to head back to the house. However, while running, he hesitates to let himself be protected by Himari and heads back to the battle, where he blocks Ageha’s finishing blow with his body. Himari, after seeing the scar on his chest, starts to lose her sanity and goes on a berserker rampage, incapacitating the Daidarabotchi and almost killing Ageha until Yuto intervenes by grabbing a twig and blocks the final blow. Suddenly, the twig starts to glow with a bright light which envelops the battlefield.

When Himari comes to following her battle with Ageha, she sees a naked Shizuku rubbing her body against Yuto to her shock, only to learn she is only healing his wound with her water, and Shizuku scolds Himari on her inability to protect Yuto. Back at the house, Shizuku explains to him and Rinko about the Demon Slayers and the Amakawa’s Light Ferry ability. Shizuku also reveals to Yuto that Himari's ancestors once killed and consumed humans, and warns him that Himari will be more prone to reverting to her kind's natural instincts should she keep fighting. Later, on their way back to the house, Yuto thanks Himari for making him remember his past and tells her about the kiss on the lake, thinking that the girl at the lake might be her. However, Himari tells him that the first time she appeared as a human was on the day of his 16th birthday, leaving Yuto puzzled about the girl's identity. Later on, Yuto trains with Shizuku at a nearby park in an attempt to awaken his Light Ferry ability but is beaten easily. In an attempt to train his composure, Shizuku calls Liz and Rinko over, with Liz flirting with Yuto to Rinko's anger. However, the composure training soon turns into a harem act and a fight for Yuto's kiss until an enraged Himari appears, which leads to her smothering his face with her breasts while triggering his allergies. The next day during their walk home after school, a mysterious white-haired girl appears and introduces herself as the Demon Slayer of the Jinguji family: Kuesu Jinguji.

After tending to an injured Himari, Yuto heads over to the hotel where Kuesu is staying to talk. After receiving a warm welcome from Kuesu, Yuto explains why he refuses to join her and declines their proposed marriage. Annoyed, Kuesu slices his cheek with a dagger and nearly kills him until Yuto's Light Ferry powers sends Kuesu back towards a window, eventually falling to her death. Yuto's memories of the promise Kuesu mentioned earlier comes to light, leaving him ashamed that he couldn't remember it before an unscathed Kuesu appears behind him, telling him that the promise they've made was the only thing keeping her alive. After she accidentally tells Yuto about wanting to fulfill his desires, Himari appears, having heard the entire thing after the barrier Kuesu put was broken. Kuesu, embarrassed, takes back her words as Himari has Aya, a spirit messenger, vomit numerous weapons for her to pick out to Kuesu's disgust. Meanwhile in a forest, numerous ayakashi lay dead and mutilated while two small ayakashi run for their lives, only for them to get eaten by a mysterious figure.

While Himari spars with Yuto in order for him to fully master his Light Ferry ability, Shuten-doji puts up a barrier all over the city which catches their attention. As they arrive in the barrier, Tama appears and fights Yuto and the girls with Kaburagi watching from the sidelines. During the battle, Tama tries to kill Yuto by choking him, which provokes Himari to awaken her demon powers but Yuto snaps her out of it just in time. Satisfied, Tama lets go of Yuto and retreats from the battle to Kaburagi's surprise. After retreating to Noihara, a surprised Yuto comes to see that the girls are finding a way to make him control his Light Ferry powers through old documents stored at the archives, which later leads Yuto into reading the documents himself to everyone's surprise. Meanwhile during her walk in the forest, Himari is confronted by Shuten-doji and Tama and fights the latter by the lake. The current situation useless, Himari eventually unleashes her demon powers as she prepares for her final battle.

The battle between Tama and Himari intensifies with Himari transforming to her ayakashi form. Yuto and the other girls soon arrive at the battle scene and they are shocked at Himari's transformed appearance. Tama soon assumes her fox form with Himari in her jaws, but Himari breaks free and deals a powerful blow to her, defeating her. However, after Yuto and the girls come down to see Himari, Himari, having lost all sense of reason, attacks them and eventually beats them, leaving Yuto the only one left. Yuto then resorts to killing Himari with the latter's Yasutsuna, but drops it at the last minute and hugs Himari, telling her to come back to him. After returning to normal, Himari and Yuto prepare to fight an enraged Shuten-doji in his oni form, where Yuto then imbues Himari's Yasutsuna with his Light Ferry ability. Using her newfound power, Himari defeats Shuten-doji for good, with a hungry Tama emerging from the rubble soon after. Following the fight, Kuesu asks Yuto to use his Light Ferry ability to fix her clothes, but could not do so to his surprise. The next day, Yuto gets an angry visit from Kisaragi, Taizo, and the Class Rep, which later leads to a misunderstanding after seeing all of the girls in his house.