In 1943 Shanghai, an explosion leads to a showdown between Communist undercover agent Lu Zhengyang and former police officer Lin Shaobai. Six years later, Lu Zhengyang, now a key member of the Shanghai Public Security Bureau, recruits Lin Shaobai, the "Shanghai Expert," to tackle major cases and thwart the plots of veteran spy Zheng Lanting. As they work together, Lu Zhengyang and Lin Shaobai build trust and become close comrades.