Set in Harbin under the puppet Manchukuo regime in 1944, Song Zhuo Wen, an underground party member disguised as a railway police officer, is mistakenly identified by the head of the spy department, Guan Xue, as her lifesaver—but is actually Zhuo Wen's long-lost twin brother, Song Zhuo Wu. To uncover the ultimate mission of Kwantung Army officer Asano Daigo in Harbin, Zhuo Wen decides to use Guan Xue to infiltrate the spy department, but the sudden appearance of Zhuo Wu plunges him into unexpected trouble. Guan Xue is cold-blooded and suspicious, cruel and ruthless, while Zhuo Wu is impulsive by nature, often unintentionally sabotaging Zhuo Wen's operations. Only through the arrangements of their superiors do the brothers cooperate and protect each other, avoiding the overt and covert struggles among their colleagues in the spy department and eventually gaining the trust of Guan Xue and the Japanese officials. Within this short year, the underground party members endure hardships to uncover and stop a secret operation conspired by the Japanese army and the Soviet Union, root out the puppet Manchukuo spies within their party, and welcome the final victory of the war.