An unidentified man is stranded on Urashima, an isolated island, with no memory of his identity or background. He recalls being a time-traveler assigned to protect a girl from harm. After hearing Rinne Ohara sing in the darkness, her voice triggers his fragmented memory of the name Setsuna, and in the absence of any clear recollections, he adopts it as his own. Rinne recognizes the name and invites him to her home, hoping he is the Setsuna she knows. As Setsuna struggles to uncover the secrets of his past and the island's history, including the local illness and the customs of the three primary houses, he begins to question the importance of his singular task of rescuing the girl he was sent to save, given the myriad difficulties the island is experiencing. With time running out, Setsuna must quickly rediscover his original purpose and carry it out.