Freshman Kaho Nikaidou is from an influential family, and as such, no one has ever said anything even remotely mean to her, for fear of incurring her household's wrath. Wishing to be around people who will not treat her as special because of her background, she leaves home to live at a dormitory for her new school year. Shortly after arriving, Kaho accidentally wanders into traffic and is saved by a tall, handsome stranger. When she asks for his name, he tells her she is weird and walks away. Having finally met the only person to ever say an unkind word to her, Kaho falls head over heels for her savior. After meeting her rescuer yet again and discovering that his name is Kanade Takahashi, she confesses her love to him. Kanade says he would like for them to be a couple, but that Kaho may not want to date him after she finds out his secret. To her shock, Kaho discovers the startling truth: Kanade is a fifth grader! Deciding that she can't date a fifth grader, Kaho intends to break up with him. But as she spends more time with Kanade, she begins to care for him even more and continues dating him. Hatsukoi Monster follows Kaho's first steps into love with Kanade, her immature, yet kind, fifth grade boyfriend. Nikaido Kaho is a daughter of a rich family and her whole life everyone's been nice to her. When moving to her new high school dormitory, she almost gets hit by a truck but is saved by a boy. It's love at first sight for Kaho but when asking him his name, he tells her she's weird. Kaho is shocked since it's the first time anyone's ever said mean things to her. Soon enough, she finds out that the boy who saved her lives in the same dormitory and so she bravely confesses her love. The boy says his name is Kanade and agrees to go out with her. Unfortunately, the following day Kaho finds out that her first boyfriend is a 5th grader.