There is a strange rumor surrounding the Urashima Tunnel: it grants the wish of those who enter at the cost of their lifespan. Kaoru Touno did not think much of this silly urban legend until he stumbled across a dark tunnel one night.

Initially dismissing the idea of going in, Kaoru is enticed by the possibility of bringing back his sister, Karen. Karen had died in an accident five years prior as a result of Kaoru's carelessness. As he enters the tunnel, what Kaoru discovers within is not quite the same as the rumors imply. The rules of the tunnel are uncertain, but that does not stop him from testing the waters.

Unbeknownst to Kaoru, his aloof classmate Anzu Hanashiro follows him on one of his expeditions. Much to his reluctance, she is intrigued by the tunnel and wants to team up with him. Although he is wary of her intentions, Kaoru accepts Anzu's offer and spends the summer with her in hopes of reviving his sister.