The story begins with Hanahi, 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 manga depicts a 12-year-old girl's innocent worries and first love.

The name of the series is 12-sai (only). Even though every volume has a subtitle, they are all the same series:

12-sai Vol. 01 - Kokuhaku (Confession)
12-sai Vol. 02 - Boyfriend
12-sai Vol. 03 - Hajimari (Beggining)
12-sai Vol. 04 - Kokoro (Heart)
12-sai Vol. 05 - Yakusoku (Promise)
12-sai Vol. 06 - Uwasa (Rumour)
12-sai Vol. 07 - Tomadoi (Lost)
12-sai Vol. 08 - Hatsukoi (First Love)
12-sai Vol. 09 - Mirai (Future)
12-sai Vol. 10 - Himitsu . Himitsu (Secret . Secret)
12-sai Vol. 10.5 (Official Fan Book)
12-sai Vol. 11 - Kataomoi (Unrequited Love)
12-sai Vol. 12 - Inori (Prayer)
12-sai Vol. 13 - Tokubetsu (Special)
12-sai Vol. 14 - Sakamichi
12-sai Vol. 15 - Takara Mono
12-sai Vol. 16 - Koino Kisetsu
12-sai Vol. 17 - Daisuki
12-sai Vol. 18 - Kizuna
12-sai Vol. 19 - Sotsugyo (Graduation)
12-sai Vol. 19.5 (Official Fan Book)
12-sai Vol. 20 - Sotsugyo-go (After Graduation)