People in the BIS suspect that Xiao Zheng Guo is either a traitor or that it isn't really him. Xiao Zheng Guo's killer is hiding inside the BIS and evidence of his betrayal is sent there. The killer poisons Lao Li to silence him. Zhou Hai Chao tries to frame Xiao Zheng Guo for the murder. Zhang Li goes to talk to Xiao Zheng Guo and he remembers some clues to the crime. (Source: jindreame at MyDramaList)