On the first day of high school, Liu Xiaoxi meets the new substitute homeroom teacher, Shi An. She later learns that Shi An is her older brother's good friend. Throughout her three years of high school, Shi An becomes her brother's mobile surveillance system. After the college entrance exam, Xiaoxi gets into the university of her dreams, while Shi An quits his job and goes abroad for further studies. But after graduation, Mu Nian unexpectedly gets together with someone right under her brother's nose.
Her brother flies into a rage: “Shi An, I'll never forgive you!”
