In the summer of 2013, 17-year-old girl Lin Kai finished her college entrance examination and went to the ancient city of Xi'an alone for her graduation trip with the support of her family. Unexpectedly, Lin Kai encountered a strange incident on the ancient city wall of Xi'an, and a series of events that followed aroused her strong curiosity. Lin Kai decided to embark on a journey to find out the truth with her only friend in her childhood, Zhao Zhu.