Shiratsunajima City was the only remaining one island and one city in Japan, but it was merged with the city on the opposite bank. Misato Tomita, a housewife who lives on the island, is at the venue of the closing ceremony of the city, and stares intently at the person on the podium. That person is Shoko Katsuragi, a novelist. Twenty years ago, she left the island and went to Tokyo, and never came back. She was a successful writer, Shoko, dressed in her urban attire, reading her sentences sentimentally with her nostalgia for her hometown. Misato was stopped by Kunikazu Miyashita who was a classmate of Shoko when she left the venue after the ceremony. Shoko also appears there, and Misato reveals her disgust. But Shoko doesn't feel bad at all. Both of them got into Kunikazu's car and headed to the house where her mother, Misato's husband and daughter were waiting. Misato thinks of her. Twenty years ago that summer, Kenichi Okudera, a young man from the city, brought a refreshing breeze to the Tomita family, a family of three who had been desperately living since their father died in an accident. And Misato exchanged a promise with Kenichi. However, her promise is not kept and she hates her older sister, Shoko. Her older sister who left the island. The younger sister who remained on the island. Twenty years later, her mother Yasue suffers from dementia. Misato has a certain suspicion. Then Shoko began to tell her surprising truth...