A man with a blood-stained kitchen knife in his hand enters the “Holmes Agency” at lunchtime and holds the knife to the neck of Kyoko, who is out to answer the door. Seiya and Ryosuke, who were with him, are aghast. Momo and Igarashi arrive at the front door shortly after and learn that they are pursuing a man suspected of murder. Momo and the others immediately left, but the man remained holed up in the office. Seiya discovers a stain on the man's chest, which he recognizes as having been made just after he had eaten Italian food. Momo, on the other hand, was convinced that the suspect was inside the office, and immediately dispatched a team of police to surround the area. Momo is so determined that she is willing to use special forces to force her way in if the situation should arise.

Seiya brings a young foreigner named Shiketa, whom he met at a certain teishoku restaurant, to the office. Shiketa is looking for a Japanese dish that is “pudgy and a little hard". Ryosuke and Kyoko look up the dish on their computer and deduce that it is stewed pork cubes. After making various flavored pork stews and having them taste them, it became clear that Shiketa was looking for a Japanese-flavored pork stew with a strange chewy texture. At the same time, Chief Yamauchi is visited by Raymond, a consular official from an oil-producing country, who asks him to urgently look for a prince who is apparently in Yokohama on a private visit to Japan. According to the story, the prince, who is the next king of Japan, has some disturbing activities around him that are aimed at his life. Momo and Igarashi, who were told that the prince is a top secret and given his portrait, immediately set out to search for him.

Igarashi is heartbroken when his beloved daughter Rei stands him up on their once-a-month date, when he receives a call that the president of a loan shark company has been murdered. The crime scene is a salaried bank office in a small building in Yokohama City. The murderer had fled, leaving behind a bag containing a commercial lunch box and a family photo of three people, including a young boy. Momo, who investigated the crime scene with Igarashi, suspects that the crime was committed by someone who was pressured by the president to repay a debt. Meanwhile, Seiya shows up at the scene and eats the lunch box as evidence, taking advantage of Momo and the others. Momo notices this and gets angry. However, Seiya, after being driven away from the crime scene, went around to lunch box shops in the area and started buying lunch boxes in the same containers. Igarashi, investigating with a photo in hand, finds the suspect, Tsujitani, looking at a Dragon Boat Festival doll in a show window.

Seiya is asked by Chief Yamaguchi, who has come to “Holmes Agency”, to take care of Momo for a while, as she is overworked. Seiya and Ryosuke hoped that Momo could cook for Kyoko, who is on a business trip, but all she could think about was the case she is currently investigating. The case was a serial arson case. Seiya, who accompanied Momo to investigate the crime scene in defiance of Yamaguchi's orders, finds curry in a charred cauldron in the kitchen, which renews his sorrow. Seiya also visited another house where the crime scene took place, and found testimony that the fire broke out while everyone was eating curry together. Soon after, a boy named Kindaichi, who was with Seiya, met his classmate Saya Endo. Saya lives near the crime scene with her mother because her father has been transferred to work alone. Her mother is also busy with work, so she is often home alone.

A middle-aged woman, Kurara Sonoda, living in Osaka, visited our office and asked us to investigate the background of a man named Yoshihiko Kameyama. According to the data brought by Kurara, Yoshihiko is a technical salaryman for an electronics manufacturer. However, Ryosuke, who was in charge of the investigation, returns with an unexpected person not long after leaving the office. It was Aika, Kurara's daughter, who had come from Osaka on the pretense of attending a training camp seminar at her employer. In fact, Yoshihiko is Aika's lover whom she is dating on the premise of marriage. Kurara, who does not approve of the marriage simply because Yoshihiko is from the Kanto region, requested a background investigation to find out what is going on with him. According to the story, Kurara's husband is a typical Kanto person, and although they love each other, they always clash because of differences in personality and the seasoning of their food.

The day after tomorrow is Kindaichi's school track meet. This month, he will run the 50-meter dash once a week in an attempt to improve his record, but Kindaichi, who hates sports, is not very enthusiastic about it. What interests Kindaichi the most is a case involving Hiroki Uehara, a cab driver and father of his classmate Susumu Uehara. In this case, Uehara encountered a gang of jewelry robbers while working, was kidnapped, and was released a day later. Seiya goes to see Uehara, who is in the hospital with Susumu, and is intrigued to hear that he was given food to eat by the criminals when he was confined. When Igarashi asked him for help in finding the place of confinement, Seiya immediately asked Uehara for more details about the situation. According to Uehara's story, the dish he ate during his confinement was sardine tsumire nabe, which was made of coarse sardines that looked as if they had been beaten with a knife and served in kombu dashi (kelp broth).

A series of soy sauce-spraying incidents targeting students at a women's college from which Kyoko also graduated. When Seiya learns that Ryosuke has encountered one of the incidents, he begins to investigate, partly because he cannot allow his precious seasoning to be used in a crime. He visits the women's college with Kyoko's help and meets the victim, Kazumi Kimura, in the cafeteria. Seiya learns that four people, including Kazumi and another who had not reported the incident, had all been victims of the Monday night attack. Seiya, who was hungry, looked at the cafeteria menu and found that, as in a women's college, everything was labeled with calories, and there was also a low-calorie “back lunch” that changed every Monday. The person in charge of everything, including seasoning calculations, is Takashi Yuasa, a staff member at Kazumi's cooking school who has been sent there for training.

Kindaichi is unable to help a classmate who is being bullied, and is devastated when Rei tells him that he lacks spirit. When Kindaichi confides in Seiya about his situation, Seiya advises him that he needs to have the courage to help those in need. Ryosuke also gave him a handmade good-luck charm, a misanga. However, Kindaichi Boy, who failed in a cram school mock exam and got pissed off, threw away Ryosuke's misanga, complained about the oatmeal that Kyoko had made with all her heart, and stormed out of the office. As soon as Kindaichi left the office, he was suddenly enveloped by a flash of light. Soon, Kindaichi realized that he was in a strange space. The streets around him and the people passing by looked just like the Edo scenery seen in period dramas. On the streets, a tile-print vendor is handing out tiles with a description of the great thief Nezumi Ko, and everyone is flocking to them.

Seiya and his friends receive an extraordinary request from the owner. The request is to steal a ring said to have been made by da Vinci from the mansion of Ichiro Tamada, the leader of the Tamada crime syndicate. For preliminary investigation, Seiya disguises himself as an electrician and enters the mansion, where he becomes intrigued by the lively party being held there. The party is a private celebration for the marriage of Ichiro's son Isao to the daughter of the Kawaguchi clan leader. In the center of the hall where the party was held, along with sake and finger foods, was the ring in a glass case. The party goes smoothly, although Ichiro's father and chairman Shigeo, who is opposed to this marriage, arrives at some point during the party. Ryosuke, in the midst of all this, confirms the laser sensors that are all over the mansion. Seiya and Ryosuke return to the office and decide to do a rehearsal using the strings.

Upon learning that Seiya had been arrested, Ryosuke and Kyoko left Lau and Kelly at the office and rushed to the police in a panic. Momo and Igarashi explained to them why they had arrested Seiya. Seiya admits that he broke into Tamada's mansion to steal a ring. Momo says that there is some evidence that this Seiya killed Shigeo, the chairman of the board. The murder took place at the same place and about the same time as the ring was stolen. A camera at the scene showed a person wearing a black jersey with the letters “I-TAN” on the back, Seiya's fingerprints were on the ceramic jug that was used as the murder weapon, and a person wearing the jersey in question was seen climbing over a wall. In addition, there was a testimony that a person wearing the jersey in question had been seen climbing over the wall. Ryosuke and Kyoko, after meeting with Seiya and learning that he insisted he was alone, reported the situation to the owner, who soon showed up at the office with Lau and others.