During the reign of Renzong Emperor, a group of European mercenaries travels to China, searching for gunpowder. A few miles north of the Great Wall, they are attacked by a monster. Only Irishman William Garin and Spaniard Pero Tovar survive. They sever the monster's arm and bring it with them. Upon reaching the Wall, they are taken prisoner by the Nameless Order, led by General Shao and Strategist Wang. The Nameless Order exists to combat Tao Tie, alien monsters that arrived from a meteorite and attack once every sixty years.[7] The Order's commanders are surprised by the monsters' first assault, which has come one week early.
Shortly after, a horde of Tao Tie assails the Wall. During the battle, Garin and Tovar are freed by Ballard, another European who came east twenty-five years earlier; detained, he now serves as an English and Latin teacher. Garin and Tovar display amazing battle skills. Saving the life of young warrior Peng Yong, they slay two monsters, earning them the Order's respect. Both sides sustain heavy losses before the monsters' queen aborts the attack. The three Europeans secretly plan to steal gunpowder and flee during the next monster attack.
That night, two Tao Tie reach the top of the Wall. General Shao is killed, placing Commander Lin in charge of the Nameless Order. Around this time, an envoy from the capital arrives with an ancient scroll that suggests that the monsters are pacified by magnets. Wang believes the stone Garin carries enabled him to slay the Tao Tie he encountered. To test the hypothesis, Garin suggests they capture a Tao Tie alive and agrees to help. This delays the escape plans, angering Tovar, who nevertheless assists Garin despite Ballard's protests.
While a new fight with the Tao Tie starts, the Westerners capture a monster and prove the theory. However, the Imperial envoy claims the creature and takes it to the capital to present to the Emperor.
It is revealed that the previous attacks had been a distraction so that the Tao Tie could dig a tunnel at the base of the Wall. While Lin investigates, Tovar and Ballard steal black powder and escape, knocking Garin unconscious for trying to stop them. Garin is arrested by the Nameless Order for his apparent betrayal and is locked up in the Wall. Some distance away, Ballard betrays and abandons Tovar. Ballard is later captured by bandits, who accidentally ignite the powder, killing the Westerner and themselves. At the capital, the envoy presents the captive Tao Tie to the Emperor. The creature awakens and reveals its position to its queen, who signals the Tao Tie to attack.
The Order rushes to the capital. Lin orders the use of hot-air balloons and sets Garin free before leaving. Wang tells him to warn the outside world, but Garin boards the last balloon with Peng and Wang. They arrive just in time to save Lin from being devoured. They land in the Emperor's palace, where Wang proposes killing the queen by tying explosives to the captured Tao Tie and giving it meat to be delivered to the queen. While approaching the site, a horde of Tao Tie attack the band, and Peng sacrifices himself to save the others.
After releasing the Tao Tie, Lin and Garin climb a tower so that Garin can detonate the explosives with an arrow. Wang sacrifices himself to buy time for Lin and Garin to reach the upper floors. Two of Garin's arrows are deflected by the Tao Tie queen's bodyguards, but Garin throws the magnet into the horde, creating a gap in the shields, allowing Lin's spear to get through. The queen is killed, and the rest of the horde dies. With the threat eliminated, Garin is allowed to return home and elects to take Tovar with him instead of a reward of black powder, much to Tovar's annoyance.
