Haruna (Haruna Amari) is a lively tomboy in the 5th year at elementary school. One day while playing with Hiroyuki, a boy whom she has known for a long time, they play a game of putting their private parts together. A sex education lesson given by a new teacher, Yagi (Kumiko Aso), makes Haruna suspect that she might be pregnant. Haruna becomes certain of it during summer holidays as her belly grows and she does not feel well. Unable to tell any adult, she decides to have the baby. Haruna's class, led by the prefect, unite to protect Haruna and her baby. Haruna manages to go through the pregnancy without the adults noticing and miraculously gives birth assisted only by the children.