Washing ashore on the remote island of Urashima without a memory of his identity, a man realizes he is a time traveler entrusted with rescuing a particular girl. Encountering Rinne Ohara, whose song triggers a faint recollection of 'Setsuna,' he adopts the name and becomes her servant, hoping to uncover his true self. Meanwhile, delving into Urashima's history, its prominent families, and a peculiar ailment confining its inhabitants, Setsuna grapples with his mission's true significance. As he unearths the island's enigmatic past and his own lost memories, Setsuna faces a moral dilemma: is altering one girl's destiny the sole purpose of his temporal journey?