Plot

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??—MAL Rewrite

The island of Urashima sits on the ocean far, far away from the mainland. The island is so lush and beautiful that one might mistake it for paradise, but the people there are trapped by a complex past and long-established traditions. Long since abandoned and all but forgotten by the mainland, the people of the island are ready to give up hope. But then, a young man named Setsuna who calls himself a "time traveler" washes up on the island. Setsuna has lost all of his memories except for a certain mission that he must fulfill. On Urashima he meets three girls, Rinne, Karen, and Sara, and begins.—Crunchyroll