..and it's flashback time. It's Teito university again, and a much younger Ishigami is sitting by himself in a lecture room. Another student behind him strikes up a conversation; this turns out to be a young, disagreeable Yugawa. Turns out they're both notorious and know of each other by reputation. There's some interesting banter about Paul Erdös and the Four-color theorem, mostly, I suspect, as a way for the author to establish a bit of local color. As they study different subjects - mathematics and physics - they don't interact much directly, but they hear of each other throughout their undergraduate days.
Returning from the flashback we have the middle-aged Ishigami and Yugawa in Ishigami's apartment, meeting each other again for the first time since they were students. Sharing takeout sushi and a bottle of spirits brought by Yugawa, they reminiscence and catch up their lives. As it turns out, Ishigami wanted to stay on for a doctorate. He was unable to do so as he needed to care for his ailing parents, leaving him with neither time nor money to continue studying. He worked as a teacher at a technical college for a few years but was so swamped with teaching duties that he was unable to do any studying or research. Also, the students were mediocre and the faculty more interested in politics and infighting than in research. He eventually gave it up and went into teaching high school instead.
Yugawa has brought a paper purportedly disproving the continuum hypothesis. Doesn't matter what it is (it's an unproven theorem about what kind of infinity the real numbers belong to) but it's a way for the author to show Yugawa and the reader that Ishigami still has it - he spends most of the night oblivious to time, finally finding the fault in the paper early next morning. Duly impressed, Yugawa follows Ishigami on the way to work. Ishigami confesses that he does frequent the bento shop where Yasuko works, and they part way as Ishigami heads that way to pick up his lunch.