The Perfect Insider

Moe Nishinosono interviews Shiki Magata, who secludes herself in the lab established by her parents on Himaka island, via video conference. At Sōhei Saikawa's office, Nishinosono meets Setsuko Gidō, whose familiarity with Saikawa annoys Nishinosono. Nishinosono later tells Saikawa, Momoko Kunieda, and Fukashi Hamanaka about the interview with Magata. Afterward, intending to meet Magata again, she suggests the campground on Himakajima island for Saikawa's upcoming seminar group trip.

En route to the island, Yukihiro Yamane tells Saikawa and Nishinosono that Magata has not interacted with other people except via video conference for the 15 years since she murdered her parents. When they visit the lab, the AI named Deborah that runs the lab's physical facilities malfunctions, and an automated cart transports a wedding-dress-clad object on a cart out of the suite, which is discovered to be Magata's corpse. In a flashback to when Magata was 13 years old, she begins a sexual affair with Seiji Shindō, her uncle and director of the lab.

Tomihiko Yuminaga declares Magata's death a murder, since her arms and legs have been amputated. With outside communication mysteriously cut off, the lab personnel wait for Shindō, who left to retrieve Magata's sister Miki, to return in order to use the radio in his helicopter to contact the police. Meanwhile, Saikawa and Nishinosono review the video surveillance footage of the outside of Magata's room and verify that no one else emerged from her suite. Shindō returns with Miki, but he is later discovered murdered in his helicopter.

The helicopter's radio has been smashed, apparently before Shindō could use it. Computer records show that no one accessed the roof between the time when Shindō and Miki arrived and when Shindō was found dead. Saikawa, Nishinosono, Yamane, and the lab security guards enter Magata's suite and discover a small robot in the bedroom. The robot's only physical capability, other than basic mobility, is to lock and to unlock the bedroom door. The robot's name is Michiru, the same as Magata's favorite doll.

In a flashback, Shindō tells Magata that his dream is to live a self-sufficient life in the mountains, equipped with only a knife. His widow Yumiko Shindō recalls for Saikawa and Nishinosono that the last words of Magata's father were "I won't let you do this." Yamane learns that Magata had scheduled an interview with a journalist for two days hence, so the lab will have another opportunity for outside communication. In another flashback, Magata buys a hunting knife and presents it to Shindō for his birthday as they have dinner with her parents.

Saikawa and Nishinosono return to the campground the next morning and tell the group about the murders. In the evening, they return to the lab. Nishinosono notices upon reviewing the surveillance video again that, when the cart emerged from Magata's suite, the elevator went from the basement to the roof. In a continuation of the dinner flashback, Magata uses the knife to kill her mother, and Shindō and Magata kill Magata's father as Yumiko arrives.