Despite her gyaru looks, Aihara Yuzu is an innocent high school girl with no experience and no prior loves. When her mother remarries, she's transferred into the prestigious "Aihara Academy" for young ladies. However, on her first day at school she happens to witness the diligent student council president, Aihara Mei, making out with her hot homeroom teacher... Yuzu returns home, unable to hid how upset it left her, when her mother announces Mei will be her new step sister, and they'll be sharing a room...?!

After Himeko declares, "I've crossed a line with the president!" 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. Meanwhile, Harumi reveals she has four tickets to an amusement park and proposes that they invite the president to all go together...

Yuzu manages to reunite with her class, and then at the hotel she tries to confess her feelings and talk about that Christmas night with Mei. However, Mei tells her to forget about that night... Tachibana Nina happens to witness their exchange, and knowing about how her twin sister feels as well as her relationship with Yuzu, Nina resolves to help fulfill her sister's destiny. The next day Sara and Nina beat Yuzu's class to Yasaka Shrine, waiting for their chance.

Yuzu promises Nina that she'll support Sara's love with her, but after learning that the girl of Sara's destiny is Mei, she can't help but feel shaken. Nina ignores all the attention she's drawing, desperately holding Yuzu back to keep her from going to Mei. During their conflict Yuzu gets a call from Sara on her smartphone, saying "There's something I need to give you." She quickly races to the overlook, but Mei isn't there, only Sara greeting her with a slightly sad smile as she returns the student ID Yuzu dropped earlier.