The following version of this book was used to create this study guide: Yokoyama, Hideo. Six Four. Farrar, Straus and Giroux, 2017. First American edition.

Yoshinobu Mikami’s daughter Ayumi had recently gone missing. Mikami worked as the Press Director, communicating police news to local reporters. He kept his missing daughter a secret from many of those around him. His wife had greatly suffered after their daughter's disappearance, and she rarely left their house, hoping to hear the phone ring. The couple had recently received three silent calls at home. Minako believed the caller had been their daughter.

Chapter One is set in December of 2002; the press were angry at a recent decision made by the Prefectural HQ to withhold the name of a pregnant woman, the driver in a fatal accident. They decided, as the Press Club, to submit a formal written protest to the station captain. Stationed at the Prefectural HQ, Mikami would not allow the reporters to bypass his authority. His immediate supervisors agreed that this protest must be kept a secret from the more executive staff - especially the captain.

Meanwhile, Mikami was assigned to contact Yoshio Amamiya, the father of a now-murdered kidnapping victim. Mikami was to ask Amamiya if he would accept a visit from the police commissioner general in order to reinvigorate the search for his daughter’s uncaught murderer. The case was 14-years old and unsolved. Though on Mikami's visit, Amamiya denied Mikami's request. Following Mikami’s failure to convince Amamiya, he began personally researching the case in hopes of finding information that could explain Amamiya's hesitation to agree to the commissioner's visit. Acting independently and secretly as a detective, Mikami had been asking countless people about the Six Four case. In talking to Akama, one of Mikami’s superiors, Mikami learned that the pregnant driver whose identity was kept secret from the police actually was the daughter of someone on an official police committee. Mikami felt foolish for his previous blind loyalty while holding his post.

In the midst of Mikami’s search for information, a gag order was imposed by Criminal Investigations, making Mikami highly suspicious of the department. What was it that was being kept secret by the police? It was recommended to Mikami that he think about why the commissioner was suddenly visiting. Mikami began to investigate what had happened at the Amamiya residence the night after the kidnapping. He decided to focus on those who working in the Home Unit, which had four members: Hiyoshi, Kakinuma, Koda, and Urushibara.

First, Mikami visited Hiyoshi’s house and found that he had quit the force and had become a recluse. He had been silently hiding in his bedroom since his work on the Six Four case. He would not even speak or show his face to his own mother. Hiyoshi thus would not directly be of help to Mikami in his quest for information. Mikami then visited Kakinuma, also a member of the Home Unit. Kakinuma was not home, but his wife directed Mikami to the area where her husband had been working. Mikami found him parked in front of a supermarket in the area that Kakinuma’s wife had directed him. From Kakinuma, he learned that Koda had quit the force and gone missing shortly after the case. The night this team was stationed in Amamiya’s house, Koda’s recording equipment failed to capture the kidnapper’s voice on tape. Urushibara (the most superior member of the Home Unit) had covered up the error. Now, this error was a secret harbored by the heads of Criminal Investigations.

Midway through the novel, news of a poorly handled suicide that occurred on police headquarters was leaked to the press. They used this as further leverage in their argument for abolishing the police’s use of anonymous reporting.

Mikami finally learned the purpose of the commissioner general’s visit from Arakida, the director of Criminal Investigations. Tokyo was sending the commissioner to announce their planned takeover of Arakida’s job position. Out of those who Mikami subsequently told about Tokyo’s plan, many argued that this job replacement would improve police relations. Mikami was initially skeptical. However, he successfully got Amamiya to agree to the commissioner’s visit.

Meanwhile another police error was leaked to the press. In Prefecture D, a sergeant had sexually assaulted a detainee and then bribed her for her secrecy. In response, Mikami decided to enact a new policy of full disclosure. The press welcomed this development, but they demanded that Mikami come clean about the pregnant woman’s true identity and were horrified to learn the truth.

The novel’s action comes to a head when Mikami finds almost the entire Prefectural HQ staff convened in the station’s assembly hall. Administrative Affairs (his department) had purposefully been excluded from the gathering. Demanding attention, Mikami was informed of a new kidnapping; it was the day before the intended commissioner’s visit. Mikami wondered if this was a hoax perpetrated by the Prefectural HQ in order to stop Tokyo from taking over the internal job, or at least from stopping the commissioner's announcement. There were already an incredible number of similarities between this new kidnapping case and that of Six Four.

It was Mikami’s job to deal with the press, informing them of developments in this new case. Denied meaningful updates from his superiors, Mikami instead asked to work on the front line, and he was allowed to ride in the police vehicle that monitored the victim’s father [Mesaki] as he drove to deliver the ransom money. As the surveillance vehicle followed the victim's father, Mikami reported live updates to the Press Club. The victim’s father was led on a chase across City D, eventually being told to park and burn the money. He did so, while police looked on. After the money was destroyed, the father got a call from his wife; their daughter had been found, arrested for theft. The father was then seen eating part of a note he found at the burning site.

Mikami and the other police in the vehicle with him had by this point though realized the truth: Mesaki was the Six Four kidnapper. Amamiya and Koda had been working together to carry out the plot to deliver Mesaki to the police. Amamiya had been silently calling numbers in a phone book, listening for the kidnapper’s voice he heard during the unrecorded call 14-years-earlier. Mikami and Minako's silent calls had been from Amamiya, not Ayumi. The novel ends without Mikami’s daughter Ayumi being found. Mikami though has committed himself to maintaining transparency with the press in his job.