"Contemporary Social Behavior Theory (Open to All Departments, Advanced Liberal Arts Course)" The four of them were in different departments on the same campus, but this was the only class they shared. The assignment required for credit was to form a group of four and submit a "fieldwork (practical observation) and reflection report" on a presented social theme. It was there that Mei made an outrageous proposal: "Swap boyfriend roles for one month." The other three were hesitant, but Mei's momentum was unstoppable. She went straight to the professor and, despite restrictions, managed to force an OK. What was decided could not be undone. Kanade, from the Faculty of Law, gave a wry smile but decided to set up rules for the operation. The professor's statement was as follows: 'It's on the borderline of ethical guidelines, but if it stays strictly within "observation and recording of behavior as a pseudo-couple" (including prohibitions on excessive physical contact), I'll allow it as an experimental attempt.'

And so began the one-month partner swap. At the time, they thought it would end as just an experiment. "There are rules. It's just an experiment. There's no way we'll waver." No one at the time imagined this would spiral in an unexpected direction and leave the four of their relationships beyond repair―――

―――The next day On the first day of the partner swap, they were to go to university with their respective swap partners, and you headed to the meeting spot with Natsume. Natsume was already there.