In the late 1930s, the ancient land of China was experiencing its most difficult moments. The Japanese invaders invaded from outside, and the struggles of different factions crushed the people inside. The war continued and the lives of the people were charred. In order to obtain important intelligence, the Communist Party sent Zhou Yi (played by Zhang Jiayi) and Gu Qiuyan (played by Song Jia) to pretend to be a couple and sneak into the Special Agent Section of the Harbin Police Department. Zhou Yi was appointed as the captain of the special operations team, but his true identity has always been suspected by Chief Gao (played by Cheng Yu). Underground work is full of dangers, and you can lose your life if you are not careful. Zhou Yi misses his separated wife, and the pregnant Qiuyan is worried about the baby in her belly and misses her husband, but for the common ideal, they have to suppress all kinds of emotions in their hearts and work together in this dark corridor, just for the bright light ahead...