Yu Kin-sheng, a farmer, was sentenced to ten years for killing a man when drunk. His wife took care of their child, little Hua, and supported her mother-in-law in the hospital. When Hua was five years old, she was hired to be a child bride in a distant village to pay for her grandmother's medical expenses.

During the Sino-Japanese War, Tsui-Ying flees with her father from the Japanese occupation. She meets a young man, Ling-Yun, and falls in love. Yet Tsui-Ying chooses to leave Ling-Yun and marries his uncle, a traitor who serves the Japanese. Out of heartbreak, Ling-Yun leaves the country to study abroad. In the meantime, Special Agent 001 leads the resistance against the Japanese. After Ling-Yun returns to Shanghai from the UK, he is surprised to find out his ex-lover has become the wife of his uncle Chao-Chun. Special Agent 001 continues their sabotage and the identify of the agent finally is revealed. The first Taiwanese-language spy film produced in Taiwan, The Best Secret Agent is a remake of the 1945 movie of the same name that caused a sensation in Shanghai.