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