Born the son of a third-rate martial artist, Jang Cheon-un descends into a rough life among street thugs after his father’s death. Everything changes when he rescues a dying old man named Mu along a riverbank. As they live together, Cheon-un experiences strange nightly dreams where the old man passes down high-level martial arts—training that rapidly translates into real-world strength. When elite enforcers from the Nine Heavens Castle suddenly track down Old Man Mu, the elder vanishes, and Cheon-un finds himself swept into the faction’s grueling training grounds, Gangryeong Valley.

Surrounded by contempt, Cheon-un proves his worth through cutthroat competition and secures a position as the personal guard to the Young Castle Lord. However, the role is less about physical protection and more about navigating deadly political conspiracies. As internal traitors and external enemies plot to seize control of the castle, Cheon-un realizes that to truly protect the Young Lord and survive, he must become absolute. Vowing not just to die for his master, but to achieve invincibility so neither of them perishes, Cheon-un draws his sword to carve out his own destiny.