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.