This story unfolds 8 years after 'Battle of Hwangsanbeol', which destroyed Baekjae, when Silla-Tang Dynasty union attacks Goguryeo.
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.
A thousand years ago, an evil spirit was kept imprisoned in a lake by the first emperor of the Chilla empire. Now unleashed, it possesses the body of Ja Woon-bi, the wife of general Biharang, with the aim to bring down the very dynasty that destroyed his people. Biharang is torn; to kill the spirit, he has to also kill his wife.
On the eighth of April of King Gyoeng-deok's 10th year of the reign of the Unified Silla Period, Guseulagi, a daughter of Yu Jong, joins the king's parade to Bulguksa Temple, where she meets a stonemason named Asadal from Buyeo. Having a crush on him, she visits Sakyamuni Pagoda, a masonic site; only to find that Asadal had broken down from exhaustion. While she takes care of him, her love for him grows. But Asadal misses his wife who is waiting for him in Buyeo. Meanwhile, Geum Seong who has a crush on Guseulagi, asks his father, Geum Ji, to propose on behalf of him. But Yu Jong is so dissatisfied with Geum Ji because he is a treacherous subject, that he turns down the proposal, instead of hurrying up his daughter's marriage with Gyeongsin, a faithful subject. Asanyeo who has been waiting for her husband Asadal in Buyeo goes to Bulguksa Temple after her father-in-law had died.
His parents captured by the Baekje armies, prince Eo-jin infiltrates the enemy castle alone to seek revenge. His plot is ruined when he falls in love with a princess there and is forced to choose between betraying his parents and nation or the woman he loves.
Once upon a time, under the reign of the three kingdoms, there was a woman who tempts a Buddhist priest named Cho. She is a one-thousand-year-old fox who intends to reincarnate as a human being. Not knowing this, Cho lives with the fox. But in the end, they get separated harboring sadness of unfulfilled love in this world.
Prince Kim Chu, last prince of the Silla Kingdom, falls in love with an enemy princess. Because this is not allowed, he must renounce his heredity and retire to Mt. Chiaksan.