In Shanghai in 1940, while Chen Jiaping, an underground party member codenamed Plum Blossom Four, was on a mission, he encountered his former fiancée Zuo Shuangtao at a critical moment when he was almost exposed.At this time, Zuo Shuangtao was already an agent of the Kuomintang military command. He failed in his assassination mission and was looking for a way out of trouble.Chen Jiaping and Zuo Shuangtao, who did not know each other's identities as agents, relied on their tacit understanding to cope with the Japanese's rigorous interrogation, and finally passed the test as a couple.Suspicious of the disguised couple, the Japanese immediately forced Chen Jiaping and Zuo Shuangtao to get married, and Zuo Shuangtao also used the trick to work in a commercial bank.After that, Chen Jiaping completed the mission while dealing with the identity crisis that could not be eliminated. He also tried to test Zuo Shuangtao's true identity. The struggle became increasingly cruel. In the end, Chen Jiaping discovered that Zuo Shuangtao was the Five of Hearts, a military reunification agent. Zuo Shuangtao also discovered that Chen JiapingIt's Agent Four of Clubs.Through an in-depth understanding of the anti-Japanese attitudes and contributions of the two parties, Zuo Shuangtao finally chose to fight with Chen Jiaping, inspired by his communist beliefs.They have gone through many tests, completed arduous tasks under the leadership of the party organization, and embarked on a new journey together.