Freshman Kaho Nikaidou hails from an influential family, meaning no one has ever spoken anything remotely unkind to her, fearing the wrath of her household. Desiring to be around people who do not treat her as special due to her background, she leaves home to live in 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 again and discovering his name is Kanade Takahashi, she confesses her love to him. Kanade says he would like them to be a couple, but warns that Kaho may not want to date him once she learns his secret. To her shock, Kaho discovers the startling truth: Kanade is a fifth grader! Deciding she cannot 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.