In Shanghai in 1940, Chen Jiaping, a Chinese Communist underground member with the codename "Plum Blossom Four," encountered his former fiancée, Zuo Shuangtao, in a critical moment while executing a mission that was on the verge of being exposed. At this time, Zuo Shuangtao had already become a KMT military intelligence agent, who had failed her assassination mission and was also looking for a way to escape. Unbeknownst to each other’s true identities as agents, they relied on their past rapport to handle the stringent questioning by the Japanese. Ultimately, they managed to pass through under the guise of a couple. Suspecting their pretended relationship, the Japanese forced them to marry, and Zuo Shuangtao cleverly used this opportunity to infiltrate a trading company. Subsequently, Chen Jiaping continued to complete his assigned tasks while managing the ongoing crisis of his identity and trying to probe Zuo Shuangtao's real identity! The struggle grew increasingly brutal, and ultimately, Jiaping discovered that Shuangtao was the KMT agent “Red Heart Five,” while Shuangtao uncovered that Jiaping was the CCP agent “Plum Blossom Four.” Through her in-depth understanding of both parties' attitudes and contributions to the anti-Japanese effort, Zuo Shuangtao ultimately chose to fight alongside Chen Jiaping under the influence of her communist beliefs. They underwent trials and completed arduous tasks under the leadership of the party organization, embarking on a new journey together.