In the year 645, the Tang Dynasty’s Emperor Li ( Park Sung-Woong) invades the ancient Korean kingdom of Goguryeo with ruthless efficiency. Li's seemingly invincible army marches from one victory to the next on their way to the ancient capital on Pyeongyang. Only the fortress of Ansi stands in their way, under the command of Yang Man-Chun ( Zo In-Sung). But Yang has other problems, namely the assassin Sa-Mool ( Nam Joo-Hyuk), dispatched by the Goguryeo high command which views Yang as a traitor. As Li's superior forces close in, Sa-Mool is moved by Yang's concern for the men and women under his command, however. The two set their differences aside and decide to defend Ansi to the death as the fate of a nation hangs in the balance.