In the year 2002, the groundbreaking Marine Express, an underwater train linking America and Japan, prepares for its inaugural trip. When the lead engineer suspects foul play during the test run, he enlists the services of private investigator Ban Shunsaku. Upon reaching the engineer's estate, Shunsaku discovers his client's lifeless body. With only a fleeting glimpse of the culprit, Shunsaku embarks on the Marine Express to unravel the enigma. Little does he know, the train harbors mysteries far more intricate than he ever anticipated.