Gai is Ling'an's top assassin. In order to avenge his brother, Gai was ordered to go to Beiqi to steal the secret notes of the deployment of troops. On the way, he met Ke and Ying. The three of them tried to steal the letter of the deputy general Jie of Beiqi. Unexpectedly, Ying had been bribed, and all this was a big conspiracy. There was no military deployment notes at all. What they were ordered to find was a recipe for elixir. Gai, who learned the truth, decided to rescue his companions, kill his enemies, and eliminate harm for the people.