Following the conquest of Gojoseon by Han China in 108 BCE, the surviving tribes and city-states of Manchuria and the northern Korean Peninsula are harshly subjugated as tributaries to the Han, who are portrayed as ruling with an iron fist from the Four Commanderies. Haemosu, the leader of the local resistance in the form of the Damul Army, covertly teams up with Prince Geumwa of Buyeo to defend and rescue Gojoseon refugees throughout the land. After being injured in a battle, Haemosu is rescued by Lady Yuhwa of the Habaek tribe (to whom Geumwa has taken a fancy), and they fall in love. Haemosu is subsequently ambushed and captured by Han forces (and after falling off a cliff is presumed dead by the outside world), and the now-pregnant Lady Yuhwa is forced to seek shelter in Buyeo, where she becomes Geumwa’s concubine and gives birth to Haemosu’s son, Jumong.
Twenty years later, the young Jumong is a weak and cowardly prince overshadowed and scorned by his elder half-brothers Daeso and Youngpo, who are vying for inheritance of the Buyeo throne from their father (the now-King Geumwa). After escaping an assassination attempt by his half-brothers, Jumong leaves the palace and by a twist of fate, encounters his father, the now-elderly Haemosu in the wild. Jumong becomes skilled in combat under Haemosu’s covert tutelage, but is unaware of their father-son relationship. At the same time, Jumong forms a close relationship with Lady Soseono of the Gyeru trading clan of Jolbon. Following Haemosu’s assassination by Daeso and Youngpo, Jumong learns the truth and vows to avenge his father and drive out the Han. He returns to Geumwa and leads the Buyeo army in a campaign against Lintun and Zhenfan Commanderies, but is reported missing in action and presumed dead following an injury in battle. Subsequently, Daeso seizes power in Buyeo by colluding with Xuantu Commandery and forces Soseono to be his queen. In desperation, Soseono weds her trading partner Wootae (not knowing Jumong is still alive). Jumong, however, is rescued by the Hanbaek tribe and nursed back to health by Lady Yesoya, whom he weds. They return to Buyeo and Jumong feigns servitude to Daeso, thereby earning his trust. With Daeso’s guard down, Jumong and his lieutenants manage to intercept and lead a large group of Gojoseon refugees into the wilds of Mount Bongye, where they establish a fortress and re-form the Damul Army, against Daeso’s wishes. In despite, Daeso holds Lady Yuhwa and Yesoya while pregnant with her son Yuri hostage in the palace. After solar eclipse, Geumwa regained power from Daeso with the help of Prime Minister. He tried to convince Jumong to go back to palace and disband Damul Army as part of the conditions given by the Prime minister in exchange for his reinstatement. Jumong refused the offer and the Prime Minister tried to eliminate him and his men.
Over the next three years, the Damul Army grows and begins uniting various local tribes, to the discomfort of Buyeo and Han. Following Wootae’s death in battle, Jumong and Soseono form an alliance and unite the five clans of Jolbon and the Damul Army into a single powerful entity, which succeeds in conquering Xuantu Commandery and establishing the Kingdom of Goguryeo. When Yesoya and Yuri are reported missing from Buyeo (and presumed dead), a grieving Jumong weds Soseono and they become king and queen of the new nation.
After ruling Goguryeo for fifteen years, Jumong succeeds in reuniting with Yesoya and Yuri (who had been living in exile after escaping from the palace). Following Geumwa’s assassination by Han mercenaries, the newly-crowned King Daeso forms an alliance with Jumong, and the combined armies of Goguryeo and Buyeo succeed in conquering Liaodong Commandery with utter annihilation of the Han army in Manchuria. With Jumong’s lifelong mission finally complete and in order to prevent internal strife due to Yuri’s return, Soseono departs from Goguryeo and heads south with the pro-Jolbon faction and her teenage sons (with Wootae) Biryu and Onjo, who subsequently found the Kingdom of Baekje on the Korean Peninsula. Buyeo eventually collapses following the battlefield death of Daeso at the hands of Jumong’s grandson Muhyul. Jumong continues battling against Han China to consolidate his realm, and dies at age 40 after passing the crown of Goguryeo to Yuri.