There was a Guo Xu spy named Liu Yungu in the Song Dynasty. He lurked in the Southern Song Dynasty for several years, but his identity was exposed because he was not an old disciple.

At that time, Liu Yungu disguised himself as a jeweler and came to Shouchun. During this period, Liu Yungu often interacted with people in the Southern Song Dynasty and gained a lot of information about the politics of the Song and Jin Dynasties. But Liu Yungu is very clever. As long as it involves political topics between the two countries, Liu Yungu will praise the Song Dynasty and despise the Jin Dynasty, thus winning the favor of the people of the Song Dynasty. In some cases, Liu Yungu also brought some actually insignificant inside information of the Jin Dynasty to the Song Dynasty, and gradually gained the trust of local generals in Shouchun.

One of the generals even wrote a report recommending Liu Yungu to the emperor. Later, investigators were sent to investigate Liu Yungu's life experience. Liu Yungu is ready. He cried out his tragic experience to the investigators. Both younger brothers succeeded in becoming officials. And I am still a man who has accomplished nothing. Hearing this, the investigators were deeply moved. Liu Yungu also said that he didn't want to stay in the Jin Dynasty and was eager to play for the Song Dynasty. Song Gaozong was also a confused king. He believed Liu Yungu's words and gave him an official position. Later, there was a conflict within the state of Jin. Liu Yungu lost contact with the joint characters above, so he had to stay in the Song Dynasty and wait for Xu Jinguo to send someone.

Later, the Song court caught Liu Yungu's servant delivering letters. This servant is not a determined man either. In the case of extorting a confession by torture, he chose to expose Liu Yungu, which led to Liu Yungu being sentenced to death and ending his spy career.