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, who have already killed the rest of them. Upon escaping they seek refuge in a cave but are then 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, Commander Lin, and Strategist Wang.

The officers have been preparing for a monster invasion which happens every sixty years, and have trained five special units: Bear Troop, Tiger Troop, Eagle Troop, Deer Troop and the Crane Troop. However, they are shocked 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 Taoties attack the Great Wall, and the soldiers respond with their own special attacks.

The Eagle Troops specialized in archery, the Tiger Troops in ballista and catapults launching burning rocks. Lin’s Crane Troop consisted exclusively of female soldiers jumping off the Wall to spear the Taoties before being pulled back with elastic cords. The heavily armed and armoured Bear Troops engaged in melee while the Deer Troop served as the cavalry. The Taoties are more intelligent and numerous than thought, and kill some of the defenders, but are repelled with William and Tovar’s assistance, which earns the soldiers’ respect. In the middle of the fight, William also saved and made acquaintance with Peng Yong, a kitchen soldier who is not skilled in fighting. Later, William and Tovar meet Sir Ballard, a European who, like them, had ventured east twenty-five years ago in search of black powder, which might be sold for a fortune back in Europe. However, like them, Ballard was also taken prisoner and has been serving as an English and Latin teacher. The three foreigners discuss plans to steal black powder from the storeroom and flee while the soldiers are occupied by battle.

During the night, two monsters manage to climb the Great Wall and kill a guard. Shao and Lin lead a platoon to investigate, but they are ambushed and Shao is killed, leaving Lin in charge. Around this time, an envoy from the capital, Bianliang, arrives with an ancient scroll, which suggests that the monsters are pacified by magnets. To test this theory, William suggests that Lin should capture a monster alive using harpoons laced with sedatives, and delays his escape plans to help. At the next battle, several monsters are ensnared, and William bravely slides down a chain to ensure that an ensnared monster is not freed by other monsters. Tovar delays his escape to assist William, to Ballard’s disappointment. The soldiers resist the monsters by fighting vertically along the Wall while tied to a chain, and by using spinning mechanical blades. As the monsters are too numerous, Lin resorts to using black powder rockets, whose existence had been a secret. After the battle, the captured monster is taken to the capital by the envoy. A short while later, a hole is discovered in the base of the Great Wall, which the monsters have used as a bypass, a reflection of their cleverness. While Lin investigates, Tovar and Ballard try to escape and knock William unconscious for resisting. William is then arrested by the Chinese soldiers after they discovered the escapade of his friends but Peng Yong pleads for William, saying he witnessed William trying to stop Tovar and Ballard from leaving. Some distance away, Ballard betrays and abandons Tovar, but is himself captured by the Khitans, who, not knowing what the barrels of gunpowder were, accidentally ignite a barrel killing themselves and Ballard. Tovar reclaims the horses but is recaptured by Chinese riders.

Knowing that the monsters are nearing the capital, Lin orders the use of an unstable technology, namely hot-air balloons, to reach the capital more quickly. Most of the balloons catch fire and the soldiers fall to their deaths. William decides to fight for the Chinese and was about to board the last balloon when Peng Yong comes up to him and asks to come along. William commends Peng Yong for being brave, unlike what everyone else sees him as. William boards the last balloon with Wang and Peng Yong, following Lin to the capital, where they find most of the city already ruined. The attack on the city happened quicker than expected because the palace officials woke the subdued Taotie up unintentionally, allowing it to communicate with its herd to direct them to the city.

On the other hand, Wang thinks of a plan to kill the queen Taotie by tying explosives to the captured Taotie from earlier and giving the Taotie meat to deliver to the queen. While transporting the Taotie, the troops are attacked by other Taoties above them, including Peng Yong. Peng Yong manages to save himself using a shield but is wounded. William, Lin and Wang decides to leave Peng Yong there, promising to come back for him. However, other Taoties broke through the door and Peng Yong saves them by detonating an explosive, destroying himself together with the Taoties.

The three people climb a tower to shoot a black powder rocket to detonate the explosives, during which Wang is killed. Two rockets fail, but the third succeeds after William throws a magnetic stone at the queen right before the rocket hits. In the aftermath, Lin is promoted to regional General, and William forgoes a reward in exchange for freeing Tovar. The two are allowed to return to Europe and a contingent of cavalry are sent to protect them.