Kingdom of Goguryeo, ancient Korea, 645. The ruthless Emperor Taizong of Tang invades the country and leads his armies towards the capital, achieving one victory after another, but on his way is the stronghold of Ansi, protected by General Yang Man-chu, who will do everything possible to stop the invasion, even if his troops are outnumbered by thousands of enemies.

Set in the 7th century AD during the Korean peninsula's division into three perpetually warring kingdoms, Once Upon A Time In A Battlefield focuses on the Shilla's king Kim Choon-chu efforts to conquer his rivals: Koguryo and Baekche. To achieve this goal, he allies with mainland China's Tang empire, the superpower of the era, and together the two polities plan to attack first the weakest Baekche at its main fortress of Hwangsanbul, guarding the only easy access to the mountainous interior. But the fortress commander General Kye Baek (Park Joong-hoon) will be resisting to the end...This comedy relays on clash between ancient time and modern language combined with irony and distance to the war.

This story unfolds 8 years after 'Battle of Hwangsanbeol', which destroyed Baekjae, when Silla-Tang Dynasty union attacks Goguryeo.

A wonderful time travel exploring. The Kingdom of Goguryeo. Oh! Goguryeo!, Oh! Balhae! A travel to find the warmth of ancient history with Do-Ohl Kim-Yong-Ok, A representative philosopher of our modern age! We couldn't help but crying when we saw the towering Heul-Seung-Gohl-Seong, the first seat of government of Ju-Mong, through out the celadon-colored sunrise... From the top of the Mt. Baek-Du to Manchuria, The Scent of Goguryeo and Balhae that has been scattered all over the wild field wake our hearts up from a long, deep sleep!

Seol Hie loses her parents and eyes to the murderous brutality of General Woo Shin-tak. She trains herself in swordsmanship and martial arts for the next two decades before setting out to even the score with Woo.