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...?!

During their first night sharing a room, Yuzu had her first kiss stolen by her step sister. Since then she can't get Mei off her mind, while Mei herself goes on as if nothing happened. Meanwhile, Mei's Grandfather, the chairman of Aihara Academy, takes Mei back to his mansion. Unable to forget the sad expression on Mei's face when they kissed, Yuzu heads to the mansion to confirm Mei's true feelings, but...?!

Yuzu realizes her feelings for Mei are love, and cries. Wanting to follow her emotions, Yuzu visits the chairman's office to try and reclaim her relationship with Mei after she went away. However, she discovers the chairman suddenly ill?! Mei comes rushing to the hospital when she's called, learning that Yuzu worked hard to save the chairman, and then she begins to open up about herself...

After confronting Mei with her feelings and kissing her in the chairman's office Yuzu isn't able to face Mei and agonizes over it. Meanwhile, the student council vice president who's also Mei's childhood friend, Momokino Himeko, becomes suspicious of what happened in the chairman's office and meets with Yuzu on their day off to discuss it. There she tells Yuzu, "She's incredibly sad. As sad as she was when the current chairman, her father, left her," leaving her in shock. Yuzu's weakly walking to the station home when Mei appears before her...

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...