Yūta Togashi is a boy who, during junior high school, suffered from "chūnibyō" (中二病^?, lit. "second year middle schooler syndrome", or "delusions of grandeur"), believing that he possesses supernatural powers and dubbing himself the "Dark Flame Master", therefore alienating himself from his classmates. Finding his past embarrassing, Yūta attempts to start off high school where he does not know anyone, free from his old delusions. This proves to be difficult, however, as a delusional girl in his class, Rikka Takanashi, learns of Yūta's past and becomes interested in him.

As the plot progresses, Rikka becomes more attached to Yūta, who, despite finding her delusions irritating and embarrassing, accepts her. He helps Rikka with a number of things, including founding and maintaining her club and tutoring her. When he joins her on her summer vacation, Yūta learns that two years prior, Rikka's father, to whom she was very close, died unexpectedly due to a terminal illness, causing her to fall into her delusions. After Yūta agrees to help Rikka search for the "Ethereal Horizon" (不可視境界線 Fukashi Kyōkaisen^?), which she believes will lead her to her father, she becomes romantically interested in him and vice versa.