Yuta has a problem. He used to be a “chunibyo,” one of the thousands of Japanese students so desperate to stand out that they’ve convinced themselves that they have secret knowledge and hidden powers. Now that he’s starting high school, he’s determined to put aside his delusions. The trouble is that Rikka, his upstairs neighbor, is delusional herself. And she knows all about his past indiscretions. “Crazy in love” is redefined as fantasy worlds collide in Love, Chunibyo & Other Delusions!
As Yuuta Togashi throws away his past delusions of grandeur in order to start high school with a clean slate, he encounters a curious girl climbing down his balcolny. On the first day of school, the same girl, Rikka Takanashi, approaches Yuuta, exhibiting her own signs of 'chuunibyō syndrome'. Having eavesdropped on Yuuta getting his delusions out of his system, Rikka declares him his soulmate, despite him having no interest in reentering his delusions. However, he is asked by the teacher to walk Rikka home, as she had only moved in with her older sister, who lives above
Rikka finds a stray cat, which she claims is a chimera, though cannot keep it in her apartment due to Touka's allergy to cats and Yuuta refuses to take it in himself. Meawhile, Yuuta finds himself nervous around the class president, Shinka Nibutani, which becomes more awkward when Rikka shows up. Later, another student named Kumin Tsuyuri inquires about the cat they found, as she had been looking for her own who ran away. Both Yūta and Kumin go to Rikka's apartment, filled with all sorts of oddities, though the cat turns out not to be Kumin's. After Touka swiftly punishes Rikka for bringing the cat in, she asks Yuuta to get rid of it, threatening to expose Yuuta's chuunibyou behaviour if he refuses. As Rikka runs off with the cat, Yuuta runs off with the audio evidence and they both try to escape Touka, but are eventually caught. Afterwards, Yuuta agrees to take in the cat.
As Yuuta becomes curious about what club to join, Rikka decides she wants to start her own 'Far Eastern Magic Society' club, roping both Yuuta and Kumin to join. After trying out a few other clubs, Rikka calls on her similarly delusional friend from the school's middle school department, Sanae Dekomori, to join as well. Despite lacking a fifth member, the teacher offers them a provisional clubroom on the condition that they clean it up and recommends them as a circle. As Yuuta denies wanting to join the club himself, he resolve soon falters when Shinka also decides to join the group.
As Yūta becomes curious about why Shinka would want to join the hobby group, she asks to go to his home on Sunday. Yūta attempts to get Rikka out of the way on that day by sending her and Sanae off on a wild goose chase. As Shinka arrives at Yūta's home, she frantically ravashes his room searching for a book of Sanae's that Yūta had in his room. Yūta soon discovers this 'Mabinogion' is in fact Shinka's journal from her chūnibyō phase, when she called herself Mori Summer, which she wanted to destroy before anyone found out about her embarrassing past. As she burns the book, revealing her true nature that she only joined the club to get rid of it, she is shocked to learn Sanae has multiple copies. With Sanae unwilling to listen to her as she doesn't believe her to be the true Mori Summer, Yuta suggests she appeal to her delusions, but ultimately cracks under the pressure. Shinka reluctantly decides to stay in the group until she can finally destroy all of the evidence.
After scoring a dreadfully low score on her maths test, Rikka is faced with her circle being disbanded if she fails her next one. Rikka tries to improve the circle's status by volunteering to clean the pool but, due to playing around, it backfires, with the maths teacher deciding Rikka will need to match the class average to keep her circle. Yūta attempts to help Rikka study, but she keeps getting distracted. After learning a bit about how lonely Rikka gets sometimes, Yūta offers to come up with an e-mail address for Rikka should she pass her test, using various bribes to get her to study hard. After the maths exam, Rikka ends up scoring relatively poorly, but manages to land just above the class average, so Yūta awards her with a new e-mail address.
Yūta's classmate, Makoto Isshiki, comes to Yūta for advice after receiving an anonymous love letter, thinking it might be from Shinka. When he comes across Rikka's clubroom to talk to Yūta, he bears witness to Shinka's true nature as she feuds with Sanae. As Makoto is promptly told by Shinka to forget what he saw, he accidentally drops his notepad, containing a ranking of the girls in school. It is soon found by one of the girls and is brought to the class' attention, which Makoto reluctantly owns up to in order to protect the other boys, unintentionally offering to shave his head in apology. After being forced to go through with it, Makoto goes to the meeting point, but the writer doesn't show. Later that night, Yūta is approached by Tōka, asking him to join her and Rikka on their summer vacation.
As Yūta and the others join Rikka on her summer vacation to visit her grandparents, he notices Rikka seems awfully quiet. As the gang goes to the beach, Yūta learns from Tōka that their father passed away from an illness three years ago, pointing out that Rikka still has not accepted her father's death and had refused to visit his grave. Wanting to help Rikka, Yūta helps her escape from the house and takes her to a place where she believed she saw her father three years ago. They arrive at the place where Rikka grew up, only to find her house long gone, as Tōka catches up with them to remind Rikka about the reality of the situation.
As Tōka tries to force Rikka to accept that both their house and their father are gone, Yūta tells her that Rikka's delusions are not intended to run away from reality. Rikka then runs off, forcing Yūta to search for her, eventually finding her on a train headed back home. On the way, Yūta explains how he started his delusional behavior. Arriving home, Yūta discovers Rikka had forgotten her key and his family is out for the night, meaning the two will be alone for the night. As Yūta goes to the convenience store with Rikka, Makoto goes on a walk with Kumin and tries asking Yūta for advice, leading him to contemplate if he's in love with Rikka. As Yūta tries to put those thoughts behind him and sleep, Rikka appears to be harboring her own feelings for him.
Wanting the club to actually do something for the culture festival, Shinka decides to put on a performance starring Yuta and Rikka, who haven't spoken to each other since the night they came back from the summer trip. As Yuta worries that Rikka is still troubled over what happened, Rikka simply assumes Yuta is possessed by some evil force and recruits Sanae to perform a ritual on him which he naturally sees through. Meanwhile, Shinka deduces that Rikka is actually in love with Yuta and tries to get her to understand her own feelings and confess to him. After various failed attempts to confess her feelings, Rikka almosts slides off the school roof but is rescued by Yuta, who she embraces.
As Rikka mentions her feelings for Yūta to Sanae, Yūta also asks Makoto for advice on confessing to Rikka. Afterwards, Yūta and Rikka try to work up the courage to confess to each other and, after some initial awkwardness, Rikka manages to confess first. Afterwards, Tōka reveals to Yūta that she will be doing restaurant training in Italy, meaning Rikka will be looked after by her mother, who was largely ignored by her following her father's death. To this end, Tōka asks Yūta to help Rikka grow out of her chūnibyō phase so she can get along with her mother. After running into Rikka's mother at the culture festival the next day, Yuta ends up telling Rikka to remove her eyepatch, which doesn't settle well with her. During the afterparty, Rikka gets on stage and performs a song her father liked, before taking off her eyepatch to reveal she is no longer wearing her contact.
As Tōka moves out of the house and sets off for Italy, Rikka has been spending the last three weeks trying to become a normal person and starts to get along with others. However, Sanae is less than happy about it, especially when Rikka announces she is disbanding the club. As Yūta and Rikka clear out the magical goods from her room to prepare for her mother's arrival, Yūta wonders if he is doing the right thing. As Rikka heads off to visit her father's grave, Sanae confronts Yūta about how Rikka had wanted him to accept her delusional side. Despite feeling the same way, Yūta ends up lashing out at Sanae instead, believing it would be no good even if he told her.
As Yūta worries about having not seen Rikka in a while, Sanae ends up discarding her other persona whilst Kumin starts impersonating Rikka. Later that night, Yūta learns that Rikka has moved in with her mother and grandparents. As Yūta starts to feel guilty, he finds a letter from his past self, prompting him to go after Rikka. Along the way, he encounters Kumin, who relays a message from Rikka about how she had previously met him two years ago, around the time of her father's death, coming to admire his chūnibyō syndrome and start her own. Arriving before Rikka, Yūta finally accepts both her and his own chūnibyō side, taking her to the 'Ethereal Horizon', giving Rikka the opportunity to properly say goodbye to her father. Rikka soon resumes her chūnibyō play, despite Yūta still finding it a little embarrassing.