Back in 1592 when the Japanese were invading Korea - then the kingdom of Joseon, rich families would hire poor men to take the place of their sons in the draft. Tasked with fighting the enemy while supporting their impoverished families back home, the proxy soldiers were known to be superior combatants. Lee plays a veteran proxy soldier who aids 18-year-old crown prince Gwanghae (played by Yeo), whose father the king fled to safety in China while ordering his young son to take over the country's defense.