In 1943, a shocking explosion in Shanghai results in a confrontation between Communist Party undercover Lu Zhengyang and former police officer Lin Shaobai, each flaunting their skills. Six years later, Shanghai is liberated, and Lu Zhengyang becomes a core member in the anti-espionage efforts of the Shanghai Public Security Bureau. Faced with severe retaliation in their anti-espionage efforts, with the support of his superior and strict screening, Lu Zhengyang boldly employs the former police officer Lin Shaobai, the "Shanghai Expert". The two team up and successively solve major cases such as the "Treasury Robbery", "Counterfeit Coin", and "Hidden Radio" planned by the veteran spy Zheng Lanting of the Shanghai Secret Services' Special Station, completely thwarting the conspiracy of the hidden enemy agents. Working on cases together, Lu Zhengyang and Lin Shaobai go from constant friction to gradually building trust, and eventually becoming comrades who live and die together.