A flashback to Yukawa and Ishigami's college days shows that they become good friends on the basis of their appreciation for difficult math problems. Both of them are in the same class until they have to separate because of their different majors: one being in physics, the other math. While at college, Ishigami is delighted to have Yukawa as a friend. Yukawa, growing his hair long at the time and having somewhat of a hippie image, is able to interact not only with Ishigami but people from other departments as well, including Kusanagi, who plays on the badminton team.

After the flashback, we find Yukawa and Ishigami back together in Ishigami's apartment. They decide to order take-out and drink sake together. Yukawa gives Ishigami a difficult math problem, a challenge to the Riemann hypothesis, which takes the whole night for Ishigami to solve. While Ishigami works on the problem, Yukawa falls asleep.

The following morning, Yukawa hears Yasuko wake up to go to work. He comments a little on Yusako's status as suspect of the case, which Ishigami deflects with feigned indifference. However, Ishigami also mentions how he reguraly buys lunch from benten-tei, the lunch box shop where Yasuko works, something he forgot to mention to Kusanagi.

Yukawa accompanies Ishigami on his way to work. They pass by the homeless shanties. Before reaching the lunch box shop, though, Ishigami is careful to bid his farewell and make sure that Yukawa doesn't follow him inside, fearing that an encounter with Yasuko might cause suspicious behavior on the part of Yasuko and Ishigami.