12-sai.: Chiccha na Mune no Tokimeki

Finally, Hanabi Ayase, Yui Aoi, Marin Ogura and the others are 6th graders! One day, the girls are talking about kissing.

At Yui's household the bath broke and she has to take bath at Hiyama Bathhouse. Even though she decided not to fall in love, she starts having feelings for Kazuma Hiyama.

Yuuto asked Hanabi to go out with him. His girlfriend convinces him to take Yui and Kazuma along with them. The two couples go on a double date at the mall.

Class 6-2 has a new student: Hanabi's childhood friend who has just came back from Tokyo, who starts having a crush for Hanabi, despite seeming to be a bit of a bully.

Hanabi and Ayumu go shopping to get a present for Yuuto's birthday, after which Hanabi will go on a date with Yuuto and give him his present. Ayumu purposefully makes her late for the date, and tries to tell her that he likes her, but is interrupted by Yuuto. Due to a misunderstanding, Hanabi ends up thinking that Ayumu hates her, which depresses her, because she had grown to want to be his friend.

Ayumu tries to take Hanabi away from Yuuto. They wind up in a race. Cocoa, who wants to be Yuuto's girlfriend, decides to help Ayumu win. Can Hanabi and Yuuto discover the truth?

After Kazuma asked Yui to walk her home, she thinks what does she like him for. The answer finds it's begin in previous year...

Hanabi and Yui try to address Yuuto and Kazuma by their names, and to make the boys do the same. In the end, only Yui and Kazuma manage to, when Hanabi and Yuuto couldn't.

Due to a misunderstanding, Hanabi and Yui fall out with each other and won't speak to each other for the next few days. In the end, Hanabi reconciles with Yui while defending the later from other girls from another school.

With the Tanabata coming along, Hanabi's homeroom teacher asked the class to write something nice about their classmates in their Tanzaku in hopes that it will strengthen the bond between boys and girls in the class. Meanwhile, Cocoa plans to sabotage the relationship between Hanabi and Yuuto again by borrowing the boys' camera to make group picture of herself with Yuuto, which fails.

After accidentally hitting Yui on the head with a football during the gym lesson, Kazuma invites her to a pool date as an apology and they invite Hanabi and Yuuto. However, Eikou and his pranksters overhear the girl's conversation and on the day of the pool date, they keep spoiling the couples' time and mood, so it's up to Kazuma to chase them away.

With Yui's cram school lessons and Kazuma's work in his family's bathhouse, Hanabi and Yuuto are left to go on a date at the summer festival.