Kaho Nikaidou is a freshman from a powerful family who has never experienced criticism due to her family's influence. In an attempt to break free from her privileged background and be treated normally, she moves into a dormitory. One day, she unwittingly walks into oncoming traffic, and a charming stranger saves her. When she asks for his name, he bluntly calls her "weird" before leaving, making it the first time that anyone has ever spoken to her frankly, sparking her interest. The man she meets again turns out to be named Kanade Takahashi. She confesses her love to him, and he reciprocates her feelings, cautioning her that he may not be the perfect match for her once she discovers his secret. Kaho is surprised to learn that he is really only in fifth grade! Initially, Kaho decides that a relationship with someone so much younger is unlikely, and she intends to end things with Kanade. However, as she continues to spend time with him, she realizes that age is just a number, and they continue to date. Hatsukoi Monster follows Kaho's journey into romantic love as she experiences the joys and difficulties of being with a caring yet immature fifth-grader.