Wang Zhi tells Tang Fan that he is the only one who can figure out who killed Duo Er La. Tang Fan goes to see Wang Xian and Zhang Gongzhu to interrogate them. Tang Fan asks to see all the files, and gets Wang Zhi in hot water with the emperor. Sui Zhou figures out that the explosive device might have belonged to Wang Xian. They sneak into his office to look for clues. They find four dates written on papers in a box. They finally get clues that Zhang Gongzhu is the culprit. (Source: jindreame at MyDramaList)