The slapstick romantic comedy centers around an ordinary 16-year-old high school boy named Yūto Amakawa who is protected by a spirit—specifically, a beautiful, sword-wielding cat girl spirit named Himari. Yūto is descended from a family that has subjugated demons since time immemorial. The charm that once protected him is now impotent, but fortunately, at that same moment, Himari appears before him as his new guardian.
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.
Yuto, Himari, and Rinko are invited by their class representative for a trip to the beach as a welcoming gift for Himari. While walking home from school, Yuto reluctantly agrees to take Himari swimsuit shopping, in which Himari goes ahead of him. While calling Rinko, rain starts to fall, forcing Yuto to hide under a bridge where Shizuku warns she will kill him if he goes on with his Demon Slayer heritage and vanishes with the rain clearing up. While at the beach, it is revealed that Himari has a fear of water, in which Rinko throws her an inflatable whale toy to float on. While the others are having fun, Himari, far from the shore, gets a startling visit from Yuto when a giant wave appears from behind and crashes onto them, in which both of them survive but are dragged down into the ocean by a set of tentacles. When Yuto comes to in a cave, he meets Shizuku, who warns him that his awakening as a Demon Slayer will destroy the balance between humans and ayakashi. Just when she is about to kill him, Himari intervenes but another mizuchi appears from behind and grabs her while Shizuku conjures up a whirlpool to drown her. Yuto quickly jumps in the whirlpool and manages to rescue her but drowns after. Himari saves him by stabbing her leg with one of Shizuku's ice spikes, causing the whirlpool to disintegrate from her blood and allowing her to kill the mizuchi inside of it. Himari tells Yuto to run while she holds Shizuku off despite her injury but Yuto refuses, telling her not to sacrifice herself for his sake. Deducing there is something wrong with them, Shizuku withdraws from the battle not before telling them that she will kill them next time and disappears.
While taking his bath, Yuto gets a startling visit from Shizuku. Rinko and Himari barge in, with Himari slicing Shizuku in half only for her to reappear behind Yuto. After getting into a brief arguement with Shizuku, Yuto passes out in the bathtub. After coming to and follwing a brief scuffle with Himari, Yuto explains to Shizuku he does not want to kill ayakashi just because he's a Demon Slayer, and claims he wants to live a normal life which inspires Shizuku to move in to everyone's shock. The next day after receiving a compliment from Yuto about her housework, Shizuku berates Himari on her inability to do anything else besides fighting, deeming her as unfeminine. Later, Yuto, Rinko, and Shizuku arrive at Café Relish, where it is revealed that Himari is working as a cat-eared maid which catches Yuto's allergies. Following an intervention from Lizlet, the café's head waitress, after Himari is caught flirting with Yuto, Himari warns him that Liz may be an ayakashi. Meanwhile, Liz grows suspicious of Yuto's constant watching, and thinks he may be out to kill her. In an attempt to save herself, she spikes his tea with magic tea leaves, but her plan is foiled when Shizuku tastes the spiked tea and gives it to Taizo, who drinks it and leaves under the tea's magic. It is later revealed that Liz is a Tsukumogami after a battle with Himari leads to the latter piercing her chest with her Yasutsuna to no effect, and when she tries to destroy her true form–an antique teacup, it falls out of her hand with Liz failing to catch it until Yuto catches it at the last second, saving her. After coming to an understanding with him, Liz apologizes for her actions, with Himari and Shizuku contemplating on Yuto's kindness. The next morning, Yuto wakes up to see Rinko, Shizuku, and Himari, whose cat ears causes him get an allergy as the three maids jump onto his bed. Back at the café, Lizlet welcomes another customer.
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.
Himari engages in a fight against Kuesu but gets cut off when the latter puts up a barrier to keep her away. Inside the barrier, it is revealed by Kuesu that Yuto's grandfather had arranged their marriage. Kuesu then kisses Yuto, causing the latter to remember the kiss by the lake where it is revealed the girl kissing him was actually Kuesu. Himari breaks through the barrier but Kuesu escapes and kisses Yuto again to her annoyance, with Kuesu claiming it is her third kiss with him. She then tells Yuto to cut his relations with the ayakashi and vanishes. At the forest, the other Ayakashi are talking about Kuesu's appearance and Yuto's powers awakening until Shizuku intervenes, telling she will kill Kuesu herself. Back at Yuto's house, Himari grow irritated at Kuesu kissing Yuto, and practices kissing in front of a mirror. Meanwhile, Kuesu engages Shizuku in an intensifying fight, which ends with her pinning Shizuku on the back of a billboard. However, when Shizuku teases Kuesu on her spells and insults her being the weakest of the twelve families, a vexed Kuesu casts a barrage of powerful fire spells, forcing Shizuku to retreat. Back at Yuto's house, Himari has trouble going to sleep over the kiss from earlier. Desperate, she asks Yuto to kiss her much to his shock. After explaining her current situation to him, she apologizes and decides to go back until Yuto grabs her wrist. Just when he was about to kiss her on the cheek, the doorbell rings and, just before Himari could tell him about it, gets kissed on the lips to Yuto's surprise as her cat ears begin showing. Back at the hotel, Kuesu reinforces her goal to obtain Yuto.
After the girls' research of Kuesu leads to them discovering that Kuesu's family rose to power as Demon Slayers using Western magic, Kuesu herself arrives to everyone's surprise, informing Yuto about their future marriage. Yuto, however, refuses to agree to it unless she can change her views about the Ayakashi. Following a brief scuffle between Kuesu and Rinko, Himari intervenes and mentions the three kisses she gave Yuto as well as the kiss she received from him to everyone's shock. Kuesu counters by telling them about the promise they have made, with Yuto clueless about what she is talking about. Later on after school, Himari faces off against Kuesu, gaining the upper hand with her agility. However, the battle is soon turned around when Kuesu starts to shoot Himari with her magic gun and casts a fire spell on the ground before Himari could attack, forcing her to use Yasutsuna as a step to escape the flames. Himari is soon hit by one of the bullets and quickly falls prey to its magic, causing her to go on a rampage. Just when she was about to kill Kuesu, she quickly remembers what Yuto had said to her back in Noihara and regains her sanity. Ashamed, Himari retreats from the battle, leaving a trembling Kuesu. Late at night, Yuto asks Shizuku where Himari has been and, after being told that she went off to fight Kuesu, an unconscious and beaten Himari appears in his bedroom to Yuto's shock.
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.
During an art project at the park, Yuto is kidnapped by Ageha at a power station. While Himari and Rinko go out to search for him, Rinko gets a surprise visit from Sasa. Meanwhile at the water fountain, Taizo and the other students get a surprise visit from Shizuku, who wants to warn Himari of something. When Himari tells her about Yuto being kidnapped, they both prepare to look when they see a tired Rinko collapses with Sasa on her back. Himari demands Sasa of Yuto's whereabouts, eventually learning that he is with Ageha. Back at the power station, an excited Ageha drinks Yuto's blood and gets intoxicated after drinking it. Himari and Shizuku quickly intervene, both preparing to fight her but Ageha refuses, saying that someone is trying to kill off ayakashi in the west. Later, a man named Kaburagi from Public Safety's Section 4 appears, and after explaining to them about the numerous cases of an ayakashi comsuming other ayakashi, attempts to recruit Yuto into his division. Yuto, however, refuses when a mysterious blond-haired child appears in front of them. As Yuto and Himari run for the exit, they see a mysterious man blocking it and, after a failed series of attacks from Himari, Sasa, and Ageha, the blond-haired child attacks Himari and bites her, revealing she is the ayakashi who is killing other ayakashi. Kuesu later arrives at the scene, identifying the two as Tamamo-no-Mae and Shuten-doji, two of Japan's three strongest ayakashi. However, just as she was about to fight them, Tama and Shuten-doji vanish. Himari then recalls Tama's words from earlier, which leaves her worried.
After a report from Aya about one of the lands she came across being ravaged by Tama, Ageha comes up with a plan to awaken Yuto's powers by having the girls dress in cosplay outfits, with Ageha herself wearing a revealing schoolgirl outfit. She then starts to seduce Yuto until Himari, wearing a cat costume, stops her. Later on, Himari tries out another katana from Noihara's waepon vault but is dissatisfied until Kuesu arrives with her damaged Yasutsuna, in which Ageha has Sasa repair it with her eye beam. At the playground, a worried Yuto tells Rinko that he still wants to settle the conflict with Shuten-doji and Tama without resorting to violence. Shizuku and Kaya later appear and reveals to them about Shuten-doji and Tama's origins and their agendas. At night, Yuto tells the girls that he still wants humans and ayakashi to coexist and proclaims he will fight Shuten-doji and Tama for that cause. At the park, Kaburagi meets with Shuten-doji and Tama, who agrees to Kaburagi's deal not to harm any humans during their plans. Late at night, Himari sets off for Shuten-doji and Tama only to find Yuto blocking her path. After he tells her that they have to stick together, Himari embraces him in tears and, just when they were about to kiss each other, Kuesu interrupts them with the other girls following suit which later leads to a series of misunderstandings.
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.