One day, Kaoru Tono learned an urban legend: if you find the Urasima tunnel and walk through it to the end, your most cherished wish will come true. However, the price is great. The dream becomes reality only in exchange for a few years of one’s life. And one night, Kaoru finds himself at the entrance to that very suspicious tunnel that people talk about so much. Looking at it, Kaoru realizes that he’s being swallowed by memories of his younger sister Karen, who died five years ago in an accident. To Kaoru’s great surprise, he isn’t alone at the tunnel at the moment. Behind him came a newly transferred student, Anzu Hanasiro, who promises to help him conduct an experiment with the tunnel. But it’s unclear why she needs it—or what she wants from Kaoru?