Aihara Yuzu, an innocent high school girl, transfers into a prestigious girl's academy after her mother remarries. On her first day she sees student council president Aihara Mei kiss her teacher! And at home, Yuzu learns that Mei is her new step sister?

During their first night sharing a room, Yuzu had her first kiss stolen by her step sister! Afterward Yuzu can't get Mei off her mind, but Mei herself goes on as if nothing happened.

Yuzu realizes the truth about her feelings for Mei. Wanting to follow her emotions, Yuzu visits the chairman's office to try and reclaim her relationship with Mei after she went away.

After confronting Mei with her feelings and kissing her in the chairman's office Yuzu agonizes over not being able to face Mei. Meanwhile, the student council vice president Momokino Himeko becomes suspicious of what happened in the chairman's office.

After Himeko claims to have crossed a line with Mei, Yuzu gets competitive and wants to cross that line too. At school, both compete to see who will eat lunch with Mei, but Mei turns them down and leaves them equally crestfallen.

Mei's father, Aihara Shou suddenly returns home. While Yuzu is surprised to meet him for the first time, she explains to Shou how hard Mei's been working for the academy.