In Shanghai in 1940, Chen Jiaping, a member of the underground Communist Party of China, code-named "Plum Blossom Four", met his former fiancée Zuo Shuangtao by chance when he was on a mission and was almost exposed. At this time, Zuo Shuangtao was already a Kuomintang military agent. After failing to carry out the assassination mission, he was also looking for a way out. The two people who did not know each other's identity as agents relied on their tacit understanding of the year to deal with the strict interrogation of the Japanese, and finally passed the border as a couple. Suspicious of the two pretending to be a couple, the Japanese immediately forced them to get married, and Zuo Shuangtao also took advantage of the situation to enter the business. After that, Chen Jiaping completed the task while dealing with the identity crisis that could not be eliminated, and also tried to test Zuo Shuangtao's true identity! The struggle became increasingly cruel. In the end, Jiaping discovered that Shuangtao was the military agent "Red Heart Five", and Zuo Shuangtao also discovered that Jiaping was the CCP agent "Plum Blossom Four". Through a deep understanding of the anti-Japanese attitudes and contributions of the two parties, Zuo Shuangtao, inspired by the belief in communism, finally chose to fight with Chen Jiaping. They have gone through tests, completed arduous tasks under the leadership of the Party organization, and embarked on a new journey together.