In the time of the Song dynasty, a pair of Western mercenaries, William and Tovar, journey through ancient China searching for the secret of black powder—a powerful new weapon. They are captured by an elite Chinese military force stationed at a colossal structure known for its grandeur and defense - the Great Wall. The mercenaries soon learn that the Wall is not just a means for keeping out human invaders, but also something far more menacing—a relentless horde of monstrous creatures known as the Tao Tei, which attack every 60 years in an attempt to ravage the world.

While imprisoned, William and Tovar meet Ballard, another Westerner who has been at the Wall for years, plotting his escape with artifacts to sell to the West. William, intrigued by the advanced weaponry and organizational skill of the Chinese army, earns their respect with his archery skills. Lin Mae, the leader of an all-female warrior division, and General Shao begin to see potential value in the foreigners in their fight against the seemingly invincible Tao Tei. A tense alliance forms as the mercenaries consider their own agendas while facing the overwhelming threat alongside the massive army, testing not only the strength of the Wall's defenses but the courage and honor of those who stand behind it.