Kim Ok-vin and Uhm Tae-woong play star-crossed lovers doomed by their fathers' epic battle for political power during the Goguryeo dynasty.

Military general Yeon Gaesomun wants to go to war with the Tang dynasty, but the pacifist King Yeongnyu opts for diplomacy and national stability, and in their battle of wills the palace council is divided between the "hawks" and the "doves."

The king is particularly proud of his older daughter So-hee for being intelligent, assertive, empathetic, and an excellent swordswoman, and he respects her opinion during their frequent discussions about matters of state and politics.

Fearing that the general's power and influence are rapidly overtaking the throne's, Yeongnyu orders Yeon Gaesomun to oversee the construction of the defensive wall Cheolli Jangseong in a remote province, which would effectively exile him.

In retaliation, Yeon Gaesomun and his co-conspirators plan a revolt; it fails because of Choong's intervention, but the crown prince becomes seriously injured after he falls off a poisoned horse.

The king unexpectedly declares So-hee as his successor to the throne, dashing Jang's hopes and causing him to finally shift his allegiance to Yeon Gaesomun.

Yeongnyu asks the Geumhwadan, a secret group of warriors meant to protect the royal family and the citizens in times of uprising and war, to investigate.

But he is only a puppet king, and Yeon Gaesomun becomes the Dae Magniji ("Grand Prime Minister"), or the de facto ruler and dictator of Goguryeo.

Believing that So-hee had died in the coup, Choong reverses his earlier stance and joins Yeon Gaesomun's cause in order to gain power, castigating himself that if only he hadn't been powerless, he might've been able to save her.