As she wraps up her work for the day, Yasuko prepares to see Kudo, who wishes to take her out to dinner in order to "celebrate" the discovery of the murderer. In addition, Sayoko, the owner of the restaurant, expresses her joy about the case being over and remarks to Yasuko that "your luck as turned," hoping that Yasuko will be able to start a new relationship with Kudo. All of this makes Yasuko feel ill at ease, as the knowledge of the truth weighs heavily upon her.

As she goes to the restaurant (the same one in which she met Togashi in Chapter 1) to meet with Kudo, she is approached by Kusanagi and Yukawa, who ask to speak with her. She feels compelled to sit down at a bench and listen as Yukawa explains what Ishigami really did to keep her from being found guilty by the police. It turns out that Ishigami killed a nameless homeless man on March 10 in order to distract the police's attention away from the real murder of Togashi that took place on March 9.

The homeless man was one among the many who had set up a shanty along the riverbank that Ishigami passed on his morning walk to work. Ishigami pretended to give the homeless man a job, telling him to sleep in the same room that Togashi had been staying in. He killed the homeless man on the night of March 10 so that the autopsy would reveal the time of death as being on that day. Further, he stole a new bike, which he made sure was brand new in order to increase the likelihood of it being reported to the police, in order to ensure that the police would assume that the murder took place on March 10.

While the police were focused on Yusaku's alibi on March 10, which was somewhat weak, they failed to focus on the fact that the body of the murder victim had not been that of Togashi. Yusaku's alibi was purposely designed to be less than the ideal so that it would draw suspicion towards her, just enough suspicion to keep the police distracted but not enough to justify charging her with murder. In this way, Ishigami disguised "an algebra problem as a geometry problem," making use of the "blind spots" due to the assumptions that the police, like his math students, would make in approaching the problem of the murder.

Yasuku is visibly shaken by these revelations since Ishigami had kept this information from her. By having the police focus on the night of March 10 as opposed to March 9, Yasku and Misato could stand up under the pressure of police questioning, which otherwise might have caused them to slip up if the questioning revolved around the night of March 9. Ishigami likewise strengthened his own resolve in the face of police questioning by committing an actual murder. Had he tried to take blame for someone else's murder, he too might have foundered in his resolve to deceive the police.

After finishing his explanation, Yukawa leaves Yasuko alone, not demanding that she turn herself in but leaving her with a choice. At this point, it is uncertain how Yasuko will react to these revelations - whether she will choose to carry on with her life or go to the police to make a confession.