The St. Joachim Academy for elite girls booked the maiden voyage of the new Marine Express submarine train, which runs along the ocean floor from the sunny California all the way to the Japanese coast. The journey seems like the perfect opportunity to enjoy the beautiful underwater views, learn a bit more about marine biology and, ultimately, relax from the usual school life. However, the idyllic travel turns into a nightmare when one of the accompanying teachers appears dead in his room. Who could have killed the most popular and beloved professor in the academy? Ranko Togawa, a grumpy student but with a great sense of responsibility, decides to investigate the events accompanied by her friend Astrid to clear the name of the main suspect in the case. Of course, it won't be that simple. As the number of victims keeps growing, the possibility of facing a serial killer becomes more real and chaos slowly takes over the train. Ranko will have to make the most of the time to find out the truth and prevent the mysterious murderer from continuing to act unrestricted, though not all the answers in this mess will be to her liking… Learn the secrets hidden in the Marine Express and discover the culprit in this story of tragedy, surprises and lots of humor!