A mystery centered around the construction of the Great Wall of China. During the Song dynasty, a few miles north of the Great Wall, William, Tovar, and three other survivors from a European mercenary group searching for black powder are pursued by Khitan bandits. Upon escaping they seek refuge in a cave, but are attacked by a shadowy monster, leaving only William and Tovar alive, the former having slashed off the monster’s hand. The next day, they stumble upon the Great Wall and are taken prisoner by Chinese soldiers, who are led by General Shao, General Lin, and Strategist Wang. The officers have been preparing for a monster invasion which happens once every sixty years. However, they are frightened upon seeing the monster’s hand, identified as a member of the Taotie species, as they believed the invasion was still weeks away. Suddenly, a wave of monsters attack the Great Wall, and the soldiers respond with archery, ballistas, and catapaults which throw burning rocks. Lin leads a company of female soldiers who jump off the Wall and are pulled back with elastic cords. The monsters are more intelligent and numerous than the soldiers’ expectations, but are repelled with William and Tovar’s assistance, which earns the soldiers’ respect. Later, William and Tovar meet Sir Ballard, a European who, like them, had ventured east twenty-two years ago in search of black powder, which might be sold for a fortune back in Europe. However, Ballard was also taken prisoner, and has been serving as an English and Latin teacher. The three men discuss plans to steal black powder from the store-room and flee while the soldiers are occupied by battle.