A guy washes ashore on Urashima, a desolate island distant from the mainland, with no recall of his identity or birthplace. What he does remember is that he is a time traveler on a quest to save a particular girl from harm. As night falls, he encounters Rinne Ohara, a girl who sings a song that reminds him of a certain name—Setsuna—and chooses to adopt it as his own. Knowing another "Setsuna," Rinne hires him as a servant in the hopes that he is the same one she recalls. Setsuna, on the other hand, continues to discover more about Urashima, hoping to uncover his long-lost history. He learns about the island's folklore, its three main families, and the rare species.