This episode mainly introduces the story that after the War to Resist US Aggression and Aid Korea entered a stalemate, the Chinese People's Volunteers relied on tunnels to hold on to the front line, and at the same time carried out sniping activities with cold guns, etc., to ensure the safety of the built positions and effectively cooperate with the truce negotiations. After the fifth campaign in 1951, the front stabilized. Under the new war situation, Mao Zedong and the headquarters of the Volunteer Army decided to fight the enemy for a long time in the form of tunnel warfare. In the process of fighting and building, the tunnel system and functions of the Volunteers continued to improve, and finally formed an underground Great Wall like an iron wall, forcing the U.S. military to return to the negotiating table. The tunnel warfare gave the Volunteers the initiative on the battlefield.