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 takes him to her household as a servant, hoping that he is the same one she remembers. On the other hand, Setsuna continues to learn more about Urashima, desiring to identify his lost past. He comes to know about the island's folklore, its three great families, and the endemic disease that prevents anyone afflicted from stepping out into the daylight. As the mysteries of his missing memories and Urashima itself unfold, Setsuna must remember his purpose and fulfill his mission as soon as possible. But, as he witnesses the myriad of troubles plaguing the island, Setsuna begins to question—is his temporal displacement merely an effort to change a single girl's fate? Urashima, an island far from the mainland. The people who live there lead carefree lives. But five years ago, the island's three great families suffered a series of misfortunes and succumbed to suspicion. The people of the island cut off all contact with the mainland and began a slow decline. The key to saving the island lies in three girls who belong to the three families. But they are bound by old traditions, and are conflicted. On that island, a lone man washes ashore. The man claims to be from the future, and he begins a solitary struggle to change the island's fate.