Bae Yong-jun and Han Ae-ri, who were in the same class in 4th grade, always got along well, and they end up in the same class in 5th grade. However, one day, a fight between a female student and a male student escalates into a confrontation between all the boys and girls in the class. In the class president election, the number of girls and boys is equal, so Mr. Bok participates in the election and chooses a male class president. As a result, the girls begin to hate Mr. Bok and their relationship deteriorates further. However, during a plant collecting trip, they are caught in a heavy rain and Mr. Bok slips and loses consciousness. The children, thinking he was struck by lightning, unconsciously panic and help each other. After that, they vow to get along.