Gojoseon has fallen to the Han Empire of China. All that is left are separate city-states that have no choice but to appease the Han Empire. In the series, the Han are portrayed as cruel, taking refugees prisoner and demonstrating their new iron weapons on them.
Hae Mosu, a legendary soldier, joins forces with Geumwa, the crown prince of Buyeo, in order to counter Han savagery, creating the Damulgun, (Damul army) a band of soldiers who defend Gojoseon refugees. Hae Mosu is injured after a skirmish against Han soldiers, and floats down a river, half alive. The princess of the Haebaek tribe (Lady Yuhwa) finds him and nurses him back to health. However, the Han are looking for Hae Mosu, and discovering that the man the princess was sheltering looked like (and was) Hae Mosu, they raze the tribe to the ground. Hae Mosu escapes just in time and travels until he meets a caravan from the tribe of Gyeh-Ru. The head merchant, who is also the ruler of the Gyeh-Ru tribe, offers Hae Mosu a job within the caravan, not knowing who the man is. The ruler, Yeon Ta-Bal, tells Hae Mosu to join him in his tent, and they introduce each other and share a conversation. News of the Haebaek tribe pops up in the middle of the conversation, and seeing the shocked look on his guest’s face, Yeon Ta-Bal begins to suspect that his guest is Hae Mosu himself. The next day, the caravan travels but stops when they hear that Yeon’s wife has gone into labor. They set up camp and So Seo-No is born and protected, as Hae Mosu defends the trade caravan against a band of thieves, reinforcing Yeon’s assumption. Yeon’s caravan has trading rights with the Han Dynasty itself, so Yeon knows of Hae Mosu’s escape, and of the great reward for his capture. At this point, he is fully convinced that his guest is Hae Mosu. Yeon decides not to turn Hae Mo-su in, because he had protected his caravan and his daughter from harm.
Hae Mosu returns to Geumwa, and they resume the skirmishes. The princess of Haebaek loves Hae Mosu, and he feels the same about her. Thus, he proposes to her, and they live together. However, as the Damulgun progress in their objective to free all refugees, the Han disguise their soldiers as fake refugees, whom the Damulgun attempt to rescue. Ambushed by the fake refugees and “ironclad cavalry,” the Damulgun with him are killed, and he is captured. Hae Mosu is subsequently tortured and blinded by the Han.
Geumwa frees Hae Mosu, who, though blind, is able to ride a horse. Due to his blindness however, he and Geumwa get separated while fleeing from the Han forces in pursuit. Geumwa believes he ultimately fails in his task to save Hae Mosu, who Geumwa sees, trapped upon a chateau, shot with arrows and presumably falling dead into the river.
The Haebaek princess has Hae Mosu’s child. She goes to the Crown Prince of Buyeo after bearing her son, Jumong. He becomes a prince of Buyeo (after the enthronement of the Crown Prince) and she a Royal Concubine upon the new Emperor’s insistence.
The series then flashes twenty years into the future, where the majority of the story takes place. Having grown up being overshadowed by Geumwa’s two sons Daeso and Youngpo, Jumong has grown into a weak and cowardly prince with a talent at womanizing. Yuhwa decides that she has had enough, and arranges for Jumong to begin training in swordsmanship and the martial arts. The man that trains Jumong is the chief jailor of an underground prison, and Jumong begins train there. Within a few episodes however, Jumong causes major trouble. First he womanizes a royal priestess, causing her to be kicked out of the palace. After that, he accidentally starts a fire in the Buyeo blacksmith workshop, exposing its existence to the wary Han nation and bringing major dishonor to Buyeo. Following these two events, he is stripped of his title as a prince and is expelled from the palace. His expulsion does little to staunch the rivalry between him and his two brothers, who send assassins to tail and kill him.
While wondering around Buyeo with nowhere to go, he encounters three common robbers named Oi, Mari and Hyobbo, who disrespect and beat him up at first, but decide to serve and treat him as an older brother once they learn his true identity. He also runs into a now twenty-one year old So Seo-No, Yeon Ta-Bal’s daughter, and establishes a relationship with the Gyeh-Ru trading clan.
Jumong also encounters a mysterious old man in the prison that he used to train in, who tells him that he used to be part of the Damulgun. Though Jumong does not know this, the old man is actually Hae Mosu, his father. After narrowly escaping the prison together after Daeso and Youngpo attempt to assassinate them, Hae Mosu trains Jumong to become an even better fighter as well as an archer, and a strong master-student bond grows between them. Hae Mosu is assassinated by Daeso and Youngpo while Jumong is away however, and Jumong does not realize the old man’s true identity until well afterwards, upon which he swears to complete the work that his father could not. Seeing that Jumong has finally become the worthy successor to Hae Mosu that he had always envisioned, Geumwa allows Jumong to return to the palace.
During Daeso’s temporary taking over the power from Geumwa, Jumong is led to marry to Ye Soya, a daughter who before had survived him from a harsh injury in a war. Meanwhile, intense rivalry breaks out between Daeso and Youngpo, who are both vieing to become the Crown Prince. Youngpo brashly makes an attempt on Daeso’s life, but Jumong saves Daeso and earns his trust. Jumong uses this against Daeso, and with the help of his three lieutenants, commandeers a group of refugees that were to be sent as slaves to the Han nation, and establishes his own settlement in the mountains. Jumong names this new settlement the Damulgun, symbolic of is revival. Over the next three years, the new Damulgun (led by Jumong) and Gyeh-ru (led by So Seo-No) cooperate to unite the nearby settlements under one banner. Through many struggles with Daeso, Youngpo, Buyeo and the Han nation, the kingdom of Goguryeo is finally established. Jumong becomes the nation’s first king; So Seo-No its queen. Jumong rules Goguryeo over 15 years. Eventually Yuri, Jumong’s son, and then Ye Soya, who both had been presumed dead return to Jumong and soon Yuri becomes the Crown Prince of Goguryeo.