Sasakawa lives in a small city in the Kanto region with his wife, Miyako. Their son attends university in Tokyo, leaving the couple to live alone. One day, Miyako's younger sister, Sakiko, pays them a long-overdue visit. She has come to help take care of Miyako, who needs surgery for a herniated disc. Upon seeing Sakiko again after so many years, Sasakawa feels both surprise and attraction. While his wife is away at the hospital, Sasakawa sets up a hidden camera he borrowed from a friend inside the house to secretly watch Sakiko. This sudden intrusion into their quiet daily life will drastically change everything for them.