Xiongnu officials, the Prince of Gou () and Yu Chang (), to assassinate Chanyu Qiedihou's half-Han advisor Wei Lü () and kidnap the chanyu's mother. The Prince of Gou and Yu Chang carried out their plot while the chanyu was away on a hunt, but someone alerted the chanyu, who quickly returned and killed the Prince of Gou in battle and captured Yu. Yu admitted to plotting with Zhang. Zhang, alarmed, informed Su, who had been unaware of Zhang's plot. Aware that the chanyu was planning on forcing him to surrender to Xiongnu, Su tried to preserve his dignity by committing suicide with his sword. Wei, who had wanted Su to surrender and become an advisor to the chanyu as well, quickly summoned doctors and was able to just save Su's life. Chanyu Qiedihou, impressed with Su's heroism, also sent messengers to care for Su's recovery, while putting Zhang and Chang under arrest. After Su recovered, Chanyu Qiedihou decided to publicly execute Yu as an example and to force the entire Han mission to surrender. Zhang became apprehensive and surrendered. Su, however, was not shaken even when Wei put his sword onto Su's neck while Zhang tried to explain that he would be greatly honoured to be the chanyu's advisor. Su rebuked him for his faithlessness to the Western Han and refused to surrender. Life in exile Unable to force Su to surrender, Chanyu Qiedihou decided to try to torture him by starvation, and so put him in a cellar without food and drink. However, for several days, Su survived by consuming wool from his coat and the snow that fell into the cellar. The chanyu was further surprised and thought that the gods were protecting him. The chanyu then exiled him to Lake Baikal and ordered him to tend a flock of male sheep, telling him that he would be allowed to go home when the sheep produced young. (It is conjectured, with some evidence based on correspondence later between Su and Li Ling that the chanyu gave Su a Xiongnu wife, and she bore him children.) During exile, Su often lacked food, as the food supplies coming from the Xiongnu headquarters were not arriving steadily. He often had to resort to eating grass roots and wild rodents. Regardless of the difficulties, however, Su always held onto the imperial staff () given to all imperial messengers, and he used