白綱島市は全国で唯一残る一島一市だったが、対岸の市に吸収合併されることに。島で暮らす主婦の富田美里（広末涼子）は市の閉幕式の会場で、演壇に上がった人物を食い入るように見つめていた。その人物は、小説家の桂木笙子（水野美紀）。20年前、島を捨て東京へ行ったきり一度も帰ってこなかった憎き姉だ。作家として成功した笙子は都会的な服装に身を包み、望郷の思いを込めた文章をセンチメンタルに読み上げていた。式が終わり会場を出たところで、美里は笙子の同級生だった宮下邦和（水橋研二）に呼び止められた。そこに笙子も現れ、美里は嫌悪感をあらわにする。だが笙子はまったく悪びれない。ともに邦和の車に乗せてもらい、母と美里の夫、娘の待つ家に向かった。
美里は想いを馳せる――。20年前のあの夏、父を事故で亡くし家族三人で必死に生きてきた富田家に、都会から来た若い男・奥寺健一（田中圭）が爽やかな風を運んできた。そして、美里は健一とある約束を交わした。しかしその約束は守られず、姉の笙子を憎むことになる。島を出た姉。島に残った妹。20年の月日が経ち、母の安江（倍賞美津子）は認知症を患った。美里はある疑念を抱く。そして笙子は驚くべき事実を語り始めた…。
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...
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