One day, high school student Kaoru Tono hears a legend about the Urasima Tunnel. They say that if you go through it, you’ll get what you want on the other side. But you’ll have to pay for it with years of your own life. Soon, the boy finds himself in front of a strange tunnel that looks suspiciously like the one the rumors are about. While Kaoru is thinking about his deceased sister, Anzu Hanashiro approaches him; she wants to check the truth of the legend together with him.