In 1943 Shanghai, an explosion sparks a confrontation between communist secret agent Lu Chengyang and former police officer Lin Shaobai. Six years later, Lu Chengyang, now a key member of the Shanghai Public Security Bureau, recruits Lin Shaobai, a “Shanghai expert,” to tackle important cases and foil the plots of veteran spy Cheng Lanting. As they work together, Lu Chengyang and Lin Shaobai build trust and become close companions.