On a remote island far from the mainland named Urashima, a man washes ashore, with no recollection of his name or homeland. What he does recall, however, is that he is a time traveler with a mission: to save a certain girl from harm. As nightfall arrives, he meets Rinne Ohara, a girl who sings a tune that reminds him of a specific name—Setsuna—and decides to use it as his own. Knowing another "Setsuna" herself, Rinne...