Queen Jiso, the regent of the Silla kingdom, meets a prisoner, Wi-hwa, with a plan to secure the kingdom's future. Two outcasts, Moo-myung and Seon-u, enter the city to search for Seon-u's family.
As Seon-u sees the prince's face, Queen Jiso's general kills him and leaves Moo-myung gravely injured. Wi-hwa begins the process of forming Hwarang, a group of elite warriors.
Wi-hwa reviews the candidates shortlisted for the Hwarang. Seon-u's father Ahn-ji asks Moo-myung to pretend to be Seon-u and introduces him to his daughter Ah-ro with his new identity.
Ah-ro helps Seon-u learn the ways of nobility so that he can fit in the Hwarang. As a few goons abduct Ah-ro. Seon-u and Queen Jiso's son, Sammaekjong, try to save her.
At the Hwarang's initiation ceremony, a surprise awaits Queen Jiso. Ah-ro gets upset about Seon-u leaving her. At the Hwarang house, Wi-hwa comes up with a unique way to assign rooms to the recruits.
As training begins at the Hwarang house, tempers flare as snide comments are passed on Seon-u's social status. Ah-ro arrives at the Hwarang house as a physician and tends to an injured Seon-u.
Seon-u tries to figure out how to be a good brother to Ah-ro. The Hwarang recruits undergo a test but not all are able to pass it. Queen Jiso is surprised to see Sammaekjong at the palace at night.
Seon-u and Ah-ro discuss their past. Sammaekjong, with a fake identity named Ji-dwi learns that wealth doesn't impress all. A refugee, Woo Reuk, joins the Hwarang house to teach music to everyone.
At Queen Jiso's request, Princess Suk-myung returns to the captial and joins the Hwarang house. Ah-ro learns the truth about Seon-u. The Hwarang recruits prepare to showcase their musical skills.
As Ah-ro learns Ji-dwi's truth, Queen Jiso plans to kill her, but Ji-dwi saves her. For further training, Wi-hwa assigns trainees to each of the Hwarang recruits.
Ah-ro tends to an injured Seon-u. As per Suk-myung's order, Wi-hwa plans a tough test for his recruits and they go hunting, which puts their lives at risk.
Ah-ro promises to keep Ji-dwi's identity a secret. Ji-dwi is surprised when Wi-hwa realises his true identity. Later, the Hwarang recruits showcase their sparring skills in front of Queen Jiso.
Queen Jiso threatens Ji-dwi. She decides to send the Hwarang and Suk-myung to the Baekje kingdom for negotiations to avoid war. Ah-ro misunderstands Seon-u on seeing him close to Suk-myung.
En route to the Baekje kingdom, the Hwarang faces dangers. Queen Jiso is heartbroken as Ahn-ji refuses to treat her. Baekje's crown prince's proposal surprises Suk-myung.
To save the lives of innocent people, Seon-u claims to the prince of the Silla kingdom in front of Baekje's crown prince and the duo spars with each other. Seon-u and Ji-dwi rescue Baekje's prisoners.
Rumours about Seon-u land him in trouble. Suk-myung confronts Ji-dwi about his identity. Wi-hwa offers guidance to Ji-dwi. Queen Jiso comes up with a plan to turn her enemies' attention toward Seon-u.
Queen Jiso has a nightmare about Sammaekjong inviting trouble. Ahn-ji tries to help the people suffering from the plague outside the capital. As Queen Jiso's health worsens, her power is questioned.
Ah-ro is made a Wonhwa. Seon-u's trainee, Dan-se attacks him with a poisoned sword, but Han-Seong, a Hwarang recruit saves him and dies in the process. His death leaves the Hwarang devastated.
Sammaekjong introduces himself to Queen Jiso's court as Jinheung, the prince of the Silla Kingdom. Seon-u wonders if he could make a better king. Suk-myung tells Jinheung her wish to marry Seon-u.
Jinheung and Seon-u have a confrontation. The Hwarang pledge their allegiance to the new king. Later, the traitors are exposed and punished for their crimes.