Furuhata Rensaburo is now a well-known detective. His unique Zhongshan suit and personal style of always riding a bicycle are talked about with great relish. This time, SP takes us back to his teenage years to see the story of his encounter with the most important person in his life, which is the starting point of his detective life. 15-year-old Rensaburo (played by Yamada Ryosuke) transferred to a small mountain area because of his parents' relationship. He loved reading mystery novels and did not take the initiative to contact his classmates. One day, the flowers in the school's small flowerbed were maliciously trampled. Rensaburo found the "culprit", and his reputation as a little detective spread like wildfire. Rensaburo simply opened a detective agency in an empty house in the back mountain. Unfortunately, the guests who came to the door asked for trivial matters. Until one day, someone came to ask for a lost treasure, and things changed. But as the investigation progressed, Rensaburo realized that there was danger around him, and everything seemed to be a trap. The person he least trusted saved him at the critical moment.