Centuries-long tensions between Joseon Korea and Ming China come to a head under the rule of King Sejong (Ahn Sung Ki). As a Chinese envoy pressures the King for more tribute, a thousands-strong army slowly makes the march towards the Joseon court. Fast running out of time, warrior Chang Kang (Hur Jun Ho) plans a counterattack revolving around the construction of a powerful weapon. Orphan girl Hong Li (Han Eun Jung) holds the key to the "Divine Weapon", but to stay out of the radar of Ming spies, she must build in secret with the help of seedy merchant Seol Joo (Jung Jae Young). With enemy forces at the gate, the fate of the nation lies in the hands of these two most unlikely of heroes.