This is a story of a boy and girl who venture into a tunnel that transcends time and space. It takes place in a small town named Kazaki that overlooks the sea. One summer morning, second-year high school student Kaoru Tono hears an urban legend of a mysterious place called the Urashima Tunnel. It’s practically something out of a fairy tale, where allegedly, if you enter the tunnel, you will be able to get whatever your heart desires in exchange for years taken away from your life. That same night, Kaoru stumbles upon a tunnel that looks strangely like the Urashima Tunnel.
Kaoru finds himself standing in front of the tunnel, thinking about his beloved sister Karen, who died in an accident five years ago. If he enters the tunnel, he might be able to get her back. One day after school, he goes to the tunnel alone to start inspecting it, but someone has secretly followed him. It turns out to be the new transfer student, Anzu Hanaki, who is different from the rest of his classmates, and appears to have taken an interest in him. They decide to work together to get what they both want, but what will it cost them...?