The premise traps you in a room with two people who swore they were done with each other, then makes that room smaller every episode. The comedy works because neither of them can afford to lose, and the romance works because they're too busy fighting to notice it's already happening. You watch them pretend the other person doesn't matter while living in the same house, sitting in the same class, and slowly running out of places to hide.

Mizuto Irido and Yume Ayai bonded over a shared love of books in middle school, but their relationship soured and they split up at graduation. Just before high school begins, their parents announce they are getting married, and the former couple become step-siblings. Now they share a home, a school and even a class. To protect their parents' marriage, they put on a friendly sibling act and set a rule: whoever steps past the bounds of proper siblinghood loses. The winner earns the title of older sibling, a point they both claim, and gets to make a request of the other. Living under one roof keeps old memories close, and the feelings they thought were finished may not be. The result is a romantic comedy about two exes forced into an awkward new kind of family.