The Divine Weapon

It's 1448, the 30th year of Se Jong's reign. The Ming dynasty's meddling into Joseon's domestic affairs goes to extremes, and Se Jong secretly plans to develop Singijeon, a rocket weapon upgraded from Goryeo's firearms technology. The Ming finds out and quickly dispatches assassins to attack the research lab, but Choi Hae San, the man in charge of the project, destroys himself along with the research materials to protect the weapon's secret. His daughter Hong Li barely escapes but loses the Chongtong Records (Singijeon's blueprints) to the Ming assassins.
As the Ming's diplomatic pressure increases on Joseon for making the Singijeon, Se Jong commands Chang Gang, the chief royal guard warrior, to take the only person with the potential to complete the Singijeon, Hong Li, safely to Sul Ju, a peddler. The son of the top engineer of Hwatong Dogam (place for making gunpowder and firearms-or a weapons factory in today's language) at the end of Goryeo dynasty, Sul Ju is an exceedingly intelligent man with the top martial art skills in Joseon. He rejects Joseon as a descendant of Goryeo migrants and lives as the head of a merchant gang. He refuses Chang Gang's command to help construct the Singijeon, but in order to save his gang from a fight with rivals, he is forced to join Hong Li in seeing the Singijeon completed.
Desperate to understand the facts of Singijeon, the Ming sends an army disguised as a large envoy to intimidate Se Jong and the court. Meanwhile, the Ming army sets out to find Sul Ju's headquarters and completely destroy the slightest possibilities of developing Singijeon. After repeated failures in the project, Sul Ju risks his life to recover the Chongtong Records for Hong Li. However, he starts to have second thoughts as his comrades are sacrificed in battles. At last, as they are moments away from the completion of Singijeon, the Ming and Yeojin alliance army of 100,000 heads for Euiju, Joseon. In order to stop the war, Se Jong hands Hong Ri, and the Chongtong Records to the Ming envoy and even Sul Ju is imprisoned by the court.