A bizarre case occurred in Tokyo. The Daruma ship carrying an underground engine sank, and the engine disappeared mysteriously. At the same time, the news that the phantom thief "Iwa" had escaped from prison put the Metropolitan Police Department on high alert. When the ship "Korea Maru" entered the port, "Iwa" disguised himself as the chief engineer and escaped. Later, the ship "Queen" sank carrying millions of dollars in gold coins, and the whereabouts of the gold coins were unknown. The young detective Miura Sanji and the strange detective Otsuji Mataemon participated in the investigation. They found many clues, such as the strange soil transported from Tsukishima, the shoe prints of "Iwa", etc. Sanji suspects that "Yan" is using an underground locomotive to conspire underground, and borrows the doctor's seismograph to investigate. And "Yan" seems to be planning a bigger crime. What is his purpose? Can Sanji and Otsuji stop him?
