The story begins with Hanabi, a sixth-grade girl who is neither an adult nor a child. She deals with issues such as accidentally witnessing her homeroom teacher kissing, and worrying about experiencing physiological changes before her friends. The series depicts a 12-year-old girl's innocent worries and first love.

12-sai.: Chicchana Mune no Tokimeki ran for 2 seasons. No new episodes are scheduled.