While giving re-make-up tests to the students in his class, Ishigami decides to be lenient: instead of having the students vainly try to the solve the problems whose answers they've memorized for the test, he asks them to write down their opinions about math. Before this time, Ishigami had been giving tests in hopes of testing student's understanding of the subect. This time around, he must give tests as a formality, making sure the sudents pass, the school administration also insisting that he do everything possible to ensure that the students pass.

After leaving the classroom, he is informed of the arrival of a detective who has come to ask him questions. It is Detective Kusanagi.

While making casual small talk, Kusanagi comments on Ishigami's re-make-up tests: "Let me guess: you like putting pretty tough questions on your test." The comment perhaps hints at Ishigami's role in the cover-up of Togashi's murder, which, from Kusanagi's perpsective, is like a difficult math test.

Rather than simply copying questions from the test book and expecting students to regurgitate their answers, Ishigami takes a different approach: he takes advantage of the "blind spots" in students' assumptions about the problems, as he explains to Kusanagi. He gives the example of presenting a geometry problem as an algebra problem.

Kusanagi asks Ishigami to provide more detail about the night of March 10, providing Ishigami with a copy of Ishigami's school work schedule in order to help him jog his memory. Realizing that he is now a suspect in the case and that he is being asked to provide an alibi, Ishigami responds with his usual poise, noting that he came home at 7:00 p.m. after Judo practice. Kusanagi then proceeds to ask Ishigami about why Ishigami would call in sick on the morning of March 10 (i.e. the night of Togashi's murder) as well as March 11, the day following the murder, when it was well-known among school staff that Ishigami rarely called in sick at all, let alone for two consecutive days in a row. Ishigami brushes this off as simple coincidence with no relevance to the murder case.

Kusanagi's questioning is not too disturbing for Ishigami, who has already anticipated this eventuality, among many others, but Ishigami can't help feeling troubled by the prospect that Yukawa may have solved the case through his powers of observation. Before leaving, he removes from his desk several papers relating to difficult mathematics problems that he's been working on.

While visiting Yukawa in his lab, Kusanagi witnesses Yukawa scolding a student for merely "looking" at the results of an experiment instead of finding "meaning" in it. Kusanagi is surprised to see Yukawa display frustration with a student.

Kusanagi then proceeds to recount his exchange with Ishigami and provides Yukawa a copy of Ishigami's work schedule. They discuss the significance of Ishigami missing work for two consecutive mornings, with Kusangi suggesting that the absence from work would've allowed Ishigami enough time to finish the cover-up. Kusanagi also presses Yukawa to candidly share with him the reason for his initial suspicion towards Ishigami, which Yukawa attributes to a feeling (or intuition, rather) prompted by his curiousity about Ihsigami's affection for Yusako.

Suddenly, another student appears in the lab. Yukawa rebukes him for describing an experiment using solid-state physics instead of particle physics, further adding that "your assumptions are your worst enemies." For Kusanagi, this sounds a lot like what Ishigami told him during their meeting at the school. When Kusanagi notes this similarity to Yukawa, specifically mentioning Ishigami's reference to "blind spots due to assumptions," Yukawa suddenly becomes very grave and distraught.

Yukawa, extremely agitated and in the midst of an epiphany related to the murder case, exclaims, "I don't believe it..." His curiosity aroused, Kusanagi asks for clarification, but Yukawa demands that Kusanagi leave. Seeing the determination in Yukawa's manner, Kusanagi resigns to leaving.