The story takes place in the 1930s, when the Communist Party's secret meeting was raided by the Kuomintang, and a large number of underground party members were unfortunately arrested. Wang Feng (played by Tony Leung), the head of the underground party, arranged an urgent task, because the important task "ace" in the party was among the arrested underground party members. The enemy also knew that they had caught the big fish, but they were unable to identify who the ace was. After investigation and research, they narrowed their targets to Zhao Biwei (played by Lin Chiling), Su Jie (played by Gillian Chung), Wang Xiafen (played by Meng Tingyi), Teacher Shi (played by Su Jin) and Xiaohong. In order to force the so-called truth from the five people, the enemy did not hesitate to use various cruel punishments in an attempt to collapse the ace physically and mentally. One lie covers another lie, who is the ace?