Lin Qingyin and Liang Yixu had known each other since childhood and gradually fell in love. Due to family reasons, Liang Yixu had to reject Lin Qingyin's confession. Lin Qingyin had been waiting for many years but failed. With the persuasion of her sister Lin Yujing and her friends, she decided not to hang her youth on a tree and find new love, and came together with her high school classmate Xu Kai. In the longest summer after the college entrance examination, Lin Yujing returned to China and planned to take Qingyin to live in her home in the United States. She accidentally met Liang Yixu. Liang Yixu, who had solved his family problems, found Yujing and expressed his desire to be with Qingyin. Yu Jing thought that Liang Yixu was too casual. He could refuse if he wanted to, and be together if they wanted to be together, so she kept preventing Liang Yixu from contacting Qing Yin. Because Xu Kai is not the most suitable person, Qing Yin is very troublesome in running this relationship and has to force herself to forget the past. Xu Kai is also cynical in some aspects. The imbalance between the two people's investment in this relationship makes this relationship in danger. This is the longest summer, but autumn will eventually come, where will Qingyin go?