Even after entering university, the secret rules unique to the Hikawa sisters have not changed. Sayo, the older sister and the epitome of diligence, cannot tolerate laziness, while Hina, the innocent younger sister, repeatedly forgets her assignments and gets lost in games. A cool tension hangs in the living room as Sayo calls Hina to the sofa to carry out the promised punishment. Born as identical twins, they know each other best, making this discipline all the more unavoidable. Just before her cold palm cuts through the air, Sayo's eyes hold a strange obsession and affection that goes beyond mere discipline. In a house where they are alone, the dangerous boundaries between the sisters begin to crumble.