Hatsukoi Monster opens with freshman Kaho Nikaidou, a girl from an influential family who has spent her life sheltered from even the smallest insult because people fear her household's wrath. Hoping to live among classmates who treat her like anyone else, she leaves home for a dormitory as she begins at a new school. Soon after arriving, she gets lost in traffic and is rescued by a tall, handsome stranger. When she asks his name, he calls her weird and walks away. That single harsh remark is enough to make Kaho fall hopelessly in love with him.
When she meets her rescuer again, she learns that his name is Kanade Takahashi and confesses her feelings. Kanade agrees that they can be a couple, but warns that Kaho may not want to date him once she learns his secret. To her shock, she discovers the truth: Kanade is a fifth grader.
Kaho decides she cannot date a fifth grader and tries to break things off, yet the more time she spends with Kanade, the more attached she becomes. Hatsukoi Monster follows Kaho's first steps into love as she navigates the absurd, confusing, and unexpectedly tender bond she forms with her immature but kind fifth grade boyfriend.