Yukawa and Kusanagi discuss Ishigami’s recent confession. Although Kusanagi has reservations about Ishigami’s testimony, his intuition is not enough to justify him rejecting the story. All he can do is accept it. Yukawa seems to be distressed and grieved at this. His manner somewhat distant and aloof, Yukawa indicates his own reservations about the matter. Furthermore, Yukawa, as we already know from Ishigami’s internal self-dialogue, has solved the case, which is why, citing the N = NP problem, he encourages Kusanagi to develop his own solution to the case rather than accept that of Ishigami's.

Noticing Yukawa’s withdrawn and sullen demeanor, Kusanagi leaves him alone. He runs into one of Yukawa's graduate students, whom he pulls aside and invites to lunch in order to get more information about Yukawa’s state of mind. The student divulges that Yukawa had recently been in the university library. Following up on this information, Kusanagi goes to the library, where he discovers that Yukawa had been examining local newspapers dating from March 10 and onward, which only leaves Kusanagi more perplexed than before.

Upon leaving the library, Kusanagi notices Yukawa leaving the university and follows after him. He follows Yukawa all the way to the Shin-Ohashi bridge, across from Ishigami’s apartment. Yukawa, aware that Kusanagi has been tailing him, leads him to the Sumida River, along Ishigami’s route to school. Yukawa recounts how he had told Ishigami that “even a cog may decide how it is to be used,” which was decisive in prompting Ishigami to turn himself in, though Kusanagi fails to grasp the significance of this.

Yukawa also divulges how he came to suspect Ishigami’s involvement in the case: while talking with Yukawa one time, Ishigami commented on how Yukawa was lucky not to have lost his hair, indicating a concern for superficial matters that normally would not have been a matter of concern for Ishigami unless he had fallen in love. Furthermore, Yukawa recounts how he noticed the pangs of jealousy that showed in Ishigami’s facial expression when Ishigami saw Kudo talking with Yasuko at her restaurant. Finally, Yukawa wishes to share information with Kusanagi as friend but only if Kusanagi agrees to keep it a secret. Kusanagi reluctantly agrees.