The body of a Chinese restaurant owner, who had been in Japan for five years, was found strangely murdered in Yokohama port. Since the autopsy revealed undigested tofu, minced meat, green onions, chili peppers, and miso paste in the stomach, the police begin to investigate a rival Chinese restaurant, believing that the victim was killed shortly after eating mapo tofu. Seiya is also asked by Chief Yamaguchi to assist in the investigation, but he is unable to identify the restaurant that makes the bean curd that matches the contents of the victim's stomach. Meanwhile, a boy named Kindaichi who comes to visit the office is mistaken for the victim's son, and he and Ryosuke are kidnapped by the culprit.