In 645, massive Tang forces, led by Emperor Li Shimin (Park Sung-Woong), invade Goguryeo kingdom. Goguryeo forces, led by Yeon Gaesomun (Yu Oh-Seong), are defeated repeatedly. Yeon Gaesomun decides to retreat to Pyongyang Castle which is the capital of Goguryeo and he orders Sa-Mool (Nam Joo-Hyuk) to kill Yang Man-Chun (Zo In-Sung). Yang Man-Chun is the lord of Ansi Fortress and Yeon Gaesomun calls him a traitor for not taking part in the war with the Tang forces. Sa-Mool travels to Ansi Fortress to kill Yang Man-Chun, but, there, he is touched by the way Yang Man-Chun cares about his people.

Meanwhile, Emperor Li Shimin and his Tang army are nearing Ansi Fortress. The fortress resides on the Tang forces' march to the capital of Pyongyang. Yang Man-Chun and his army decide to defend Ansi Fortress to death.