The story takes place in the early days of the Republic of China. Liao Sicheng (played by Liu Xiaofeng) and Fang Tianyi (played by Zhang Ruoyun) were adopted sons adopted by wealthy businessman Gu Hansen. They grew up with Gu Hansen's biological daughter Gu Ting (played by Wu Ting), and have a deep friendship with each other.After graduating from Whampoa Military Academy, Liao Sicheng went to Japan for further studies. During this period, he met the beautiful Japanese woman Yumi Oshima (played by Zhang Wenqi). The two knew each other and fell in love. Unfortunately, the September 18th Incident broke out. Liao Sicheng bid farewell to his lover and returned to his motherland.On the other hand, the brave and intelligent Fang Tianyi is deeply loved and trusted by Dai Li (played by Wu Xiubo). Dai Li gave him the code name "Black Fox". Hidden under this mask, Fang Tianyi, with the help of Gu Ting and his colleague Yu Mei (played by Li Man), fought wits and courage with the Japanese and traitors. But in fact, Yu Mei, like Liao Sicheng, is an underground party member affiliated to the Communist Party of China.