Uketsu receives an email from a woman named Yuzuki Miyae, who claims to know the exact house that Uketsu wrote about. Uketsu suspects that the woman must have firsthand experience of the house’s floor plan since he didn’t include any identifying details that would allow readers to locate the house. Uketsu writes back to Miyae and learns that she is an office worker from Saitama Prefecture. Miyae wants to share what she knows about the house but is only willing to talk with him face-to-face. Uketsu is initially reluctant to meet her because he is unsure what her relationship to the couple who lived in the house might be. However, he concludes that he cannot solve the mystery without hearing what she has to say.

Uketsu and Miyae meet at a café. Miyae is a woman in her mid-twenties and appears perfectly ordinary to Uketsu. When they start talking, Miyae shares her suspicion that her husband, Kyoichi Miyae, was murdered in the house. Three Septembers earlier, Kyoichi went to visit a friend’s house but never returned. His remains were only discovered a few months earlier in Saitama Prefecture, albeit with his left hand missing. There were no clues to suggest why the hand was severed, which has made Miyae obsessed with finding out everything she can about Kyoichi’s murder. When she read Uketsu’s article, she immediately connected it to her husband’s death.

Uketsu points out that the house he wrote about was only built one year ago, after Kyoichi disappeared. In response, Miyae shows Uketsu the floor plan of a second house, which she suspects the family must have lived in before they built the house that Uketsu wrote about. Miyae found the floor plan online while looking for similar house listings. The house the floor plan represents is only 20 minutes away from her address in Saitama.

The floor plan reveals another two-story house with an asymmetrical design. The first-floor entrance leads to a toilet and living room. The living room connects to a kitchen, a hallway, and an undesignated room with a triangular shape. The hallway contains the stairs to the second floor. It also leads to a dressing room with an en-suite bathroom and another undesignated space. The second floor contains a bedroom with two single beds. The bedroom connects to a windowless child’s room, which contains an en-suite toilet. Miyae points out that the bathroom on the first floor also has no windows. Moreover, the undesignated space connected to the dressing room is reminiscent of the dead space in the Tokyo house. It is located directly under the child’s room, which would make it possible for the child to use it as a passage to access the dressing room. Following Kurihara’s interpretation, the child could then access the bathroom and kill the guest while they were in the bathtub. Uketsu starts to believe that Miyae’s claims may be true.

The house went on sale in March 2018, around the same time that the Tokyo house was built. However, the Saitama house is no longer listed, as the realtor reported that it burned down several months ago. Miyae is curious about the purpose of the undesignated triangular room, and Uketsu asks to share the floor plan with Kurihara. Before they part ways, Uketsu asks if Kyoichi was in conflict with anyone before he disappeared. Miyae does not recall any trouble and expresses her wish that the murderer tell her what really happened.

Reviewing the materials on the Saitama house, Uketsu learns that the family inhabited it for two years before moving to Tokyo. He compares it to the Tokyo house, which was sold one year after its construction. Uketsu admits that he wasn’t fully convinced by Kurihara’s interpretation when he first heard it, but the Saitama house is making him think twice. He looks up a news report about the discovery of Kyoichi’s corpse and notes that only the left hand is described as having been severed. This suggests that the rest of the corpse was intact, so he wonders whether the murders were performed by the same person.

Uketsu observes a key difference between the two houses. The Saitama house has no garage, which means the owners had no way to transport the corpses out for disposal. On the other hand, the fact that the family did not have to move the corpse through hidden passages would explain why they didn’t dismember Kyoichi’s corpse after killing him.

The morning after Uketsu sends Kurihara the floor plan, Kurihara invites Uketsu to his apartment to talk. Kurihara’s apartment is small but littered with architecture books and mystery novels. Kurihara is certain that the triangular room in the Saitama house was built as an extension to the original house. He attributes the room’s triangular shape to the larger trapezoid shape of the lot. By building a triangular room, the owners of the house effectively blocked access to what remained of the garden space. This also eliminated the possibility of building anything over the garden space, which leads Kurihara to suspect that the owners may have been hiding something in the garden space, like a cellar. The existence of a cellar would enable the family to dispose of the corpses without having to transport them elsewhere. Because the inaccessible garden space is located beside the mystery space and the child’s room, Kurihara posits that the mystery space also contained a passage to the cellar.

Kurihara suggests that when the family decided to put the Saitama house up for sale, they removed the bodies from the cellar, hid them elsewhere, and sealed off the cellar to prevent the realtors or future owners from uncovering it. This leaves Kurihara with one question, however: The construction of the triangular room would have drawn plenty of attention, which would have risked their operation. Their motivation for building the room would have had to have been greater than the risk it posed.

Over lunch, Uketsu shares his plan to visit the Tokyo house. He wants to find any evidence he can to support Kurihara’s interpretation so that they can involve the police and get the investigation off their own hands. Kurihara is worried that the owners would have thoroughly cleaned the house before putting it up for sale. He suggests that Uketsu focus on the evidence they already have. Comparing the two houses, Kurihara draws Uketsu’s attention to the differences between them. The Tokyo house has more windows than the Saitama house, implying that the owners wanted passersby to look inside. Unlike the Tokyo house, the Saitama house has no vestibule leading to the child’s room. Finally, the couple slept on two single beds in Saitama, but switched to a double bed when they moved to Tokyo, implying a change in their relationship. Kurihara wonders what caused these differences and changes his mind regarding Uketsu’s plan to visit the Tokyo house.

Uketsu emails a summary of his discussion with Kurihara to Miyae, who asks Uketsu if they can meet again. The day they are scheduled to meet, Uketsu goes out early to visit the Tokyo house. While he is looking at the house, a woman informs him that the former residents, the Katabuchis, moved out very suddenly. The Katabuchis had a son named Hiroto, and when they moved out, Hiroto couldn’t have been more than two years old. His mother often took him outside for walks, which does not fit with Kurihara’s theory that the child was being imprisoned or used to kill guests. Uketsu asks the neighbor if she noticed anything strange before the Katabuchis left. She recalls something her husband saw three months before the move. Late one night, he saw a child standing in the window of the Katabuchi house. The boy was about 10 years old and pale. The owner of the house denied that any such boy lived in the house when the neighbor asked about it the following morning.

When Uketsu shares what he has learned with Kurihara, Kurihara declares that the existence of two children resolves all the holes in his theory thus far. In 2016, the Katabuchis moved into their new Saitama house, and the wife gave birth to Hiroto the following year. The older child, whom Kurihara calls X, was already residing in the second-floor child’s room at the time. It is likely that X was adopted or not the Katabuchis’ son at all. Since Yanaoka’s realtor accepted the Katabuchis’ claim that they were a family of three, X was likely not registered as a member of their family. Kurihara speculates that X was mistreated to coerce him to commit murder. When Hiroto was born, the couple became concerned about raising him amid such violence. A triangular nursery was thus built to sequester him from the murders.

Uketsu suggests that the Katabuchis should have changed their ways if they were concerned about Hiroto’s upbringing. This leads Kurihara to speculate that someone was forcing the Katabuchis themselves to kill people. For unknown reasons, the Katabuchis moved to Tokyo in 2018 and built the house to raise Hiroto while continuing to kill. Kurihara describes the vestibule in the Tokyo house as the threshold between the house’s two faces—light and dark. The vestibule ensured that X and Hiroto would remain separated so that the presence of one would never threaten the other. This would also explain why the couple used a double bed in the Tokyo house and why there was no privacy between the bedroom and the dressing room. The layout was not intended for the couple, but for the mother to watch over Hiroto. Meanwhile, the father occupied the first-floor bedroom to protect the family from any intruders.

Kurihara recalls the neighbor’s account of X standing at the window. Based on the floor plan, it is likely that he was sitting on the double bed, which begs the question of how he got out and what happened because of it. It is possible that this incident influenced the Katabuchis’ decision to move out shortly after.

Before Uketsu’s meeting with Miyae, Kurihara reveals that he looked up more information on Kyoichi and discovered that he was a bachelor. Uketsu thus approaches the meeting with caution, though he shares Kurihara’s new interpretation of the two houses with Miyae. When he explains how the Katabuchis suddenly moved out of the Tokyo house, Miyae becomes deeply disturbed and steps away to weep. Upon her return, Uketsu reveals what he knows about Kyoichi. Miyae reveals that her real name is Yuzuki Katabuchi and that her sister, Ayano, owned the Saitama and Tokyo houses, alongside her husband.

Yuzuki explains that she was the youngest of two daughters, her sister Ayano being two years older. Yuzuki loved Ayano for her beauty and kindness. In 2005, when Yuzuki turned 10 years old, Ayano was suddenly expelled from the family. Yuzuki’s parents refused to explain why, and Yuzuki grew up missing her sister, whose disappearance her family never fully healed from. Her father stopped working and experienced alcohol addiction. He later died in a car accident in 2007. Yuzuki’s mother soon remarried a man named Kiyotsugu.

As an angsty teenager, Yuzuki often clashed with her mother. She left home after finishing secondary school in 2014 and started working an office job in Saitama. Life quieted down until October 2016, when Yuzuki received a letter from Ayano. In the letter, Ayano expressed regret over their estrangement, as well as her wish for them to reunite someday. The letter contained Ayano’s phone number, which Yuzuki called at once. Yuzuki thus learned that Ayano was married to a man named Keita and still living in Saitama under her maiden name. They talked frequently over the phone, though Ayano refused to address the topic of her expulsion from the family. When Yuzuki asked if they could meet in person, Ayano similarly declined without explanation. Yuzuki was hurt when Ayano off-handedly mentioned giving birth, as she had not mentioned her pregnancy in the past. 

A period of silence then ensued, ending in May of 2019, when Ayano invited Yuzuki to visit her and her family in their new home in Tokyo. Yuzuki was in awe of how perfect their family was. Nonetheless, a few odd details stood out to her, suggesting that they were hiding something. They prevented her from going upstairs, indicating that the stairs were under repair. Keita was also preoccupied with something, though Yuzuki never found out what it was. After the visit, Yuzuki didn’t hear from Ayano for two months. Worried for her sister, Yuzuki decided to visit them again. By then, the house was empty, the Katabuchis having moved out just weeks earlier.

Unable to reconcile the mystery surrounding her sister’s life, Yuzuki turned to her mother for answers. However, her mother refused to explain anything, let alone allow her into her house. Yuzuki revisited Ayano’s first letter and used it to locate the Saitama house. When she visited the house, she found an empty lot. She was about to give up on her investigation when she saw Uketsu’s article, recognizing Ayano’s Tokyo house at once. She also connected the detail of the dismembered murder victim to the report she once read about Kyoichi Miyae’s murder. She got in touch with Uketsu to uncover new clues about Ayano, but she was worried that Uketsu would be cautious with her if she told him her true identity.

Uketsu accepts Yuzuki’s story and concludes that everything stems from Ayano’s initial disappearance. Yuzuki remembers that the week before Ayano’s expulsion, one of her cousins died under mysterious circumstances. Uketsu invites Yuzuki to meet Kurihara and listen to his insights. They convene at a rental conference room, but Kurihara is cautious of Yuzuki. After hearing Yuzuki’s story, Kurihara asks her to continue the story of what happened to her cousin.