Natsu e no Tunnel, Sayonara no Deguchi S01

Kurousu Touno has heard a rumor: the law of space and time has no meaning for the Urashima Tunnel. If you can get it, go through it and find your heart's desire... but at the cost of your years of life. One night, Kurousu sees himself standing in front of a tunnel that looks eerily like the rumors. He remembers Karen, his sister who died five years ago in an accident. In Kurousu's utmost surprise, he is pursued by a new transfer student, Anzu Hanashiro, who offers to help him investigate the mysterious tunnel - but what does Anzu want from Kurousu? And what will he have to give up after completing the tunnel?