Natsu He No Tunnel, Sayonara No Deguchi (GAGAGA Bunko) [Light Novel]

[Machine Translation] Do you know what the "Urashima Tunnel" is? If you enter there, you can get anything you want, but instead you get old. Kaoru Tohno, a high school student, hears such an urban legend, and by chance discovers a tunnel that looks like it that night. -If she enters this tunnel, she may be able to bring back her sister who died five years ago. After school, Kaoru begins to examine the tunnel on her own, but is discovered by a transfer student, Anzu Hanashiro. The two form a cooperative relationship to get what they both want... A summer full of surprises that no one has ever experienced before begins.

「ウラシマトンネルって、知ってる?そこに入れば欲しいものがなんでも手に入るんだけど、その代わりに年を取っちゃうの―」。そんな都市伝説を耳にした高校生の塔野カオルは、偶然にもその日の夜にそれらしきトンネルを発見する。―このトンネルに入れば、五年前に死んだ妹を取り戻すことができるかも。放課後に一人でトンネルの検証を始めたカオルだったが、転校生の花城あんずに見つかってしまう。二人は互いの欲しいものを手に入れるために協力関係を結ぶのだが...。かつて誰も体験したことのない驚きに満ちた夏が始まる。