Joseon, around 1633. Lee Gyeom (Lee Joon-gi)

is the son of the virtuous nobleman Lee Won-ho, who is the king's trustworthy supporter and brother, and a central member of the secret organization Cheonwoohoe, composed of other five important nobles with the king as their leader. When a blind fortune teller, looking at Lee Won-ho's home, refers to the king that he sees a person as bright as the sun who would be adored by the people, the king killed Lee Won-ho as he believed that 'two suns cannot exist on the same sky'. Gyeom, from the inside of a safe, manages to survive and witnesses the murder; later, when he is forced to throw a rock at his mother's head to prove that he isn't her son, he loses his memory for the shock and is adopted by a retired thief, Soe-dol, who renames him "Yong". Thirteen years later, Yong regains his memories and begins to search for his older sister Yeon, only to see her being sentenced and hanged.

The murder of his older sister leaves Gyeom seeking revenge and, with the emblem on the killer's sword he remembers from thirteen years ago as his only clue, he swears to avenge his family. To find the sword and its owner, he disguises himself as the mysterious, black-swathed thief Iljimae, who breaks into the nobles' estates the members of the secret organization, Cheonwoohoe. Meanwhile, he also helps the people with injustices, becoming a hero. At the scene of each robbery, he leaves a handkerchief portraying a branch of red plum blossoms, symbol of the house where he lived and of his childhood memories: the very name of Iljimae reflects this, as "il" means "one", "ji" means "branch" and "mae" means "plum tree". The king and the nobility try to catch Iljimae and find his identity, especially the guard Byeon Si-hoo, who sees this as an opportunity to redeem himself from his life of misery and become a noble. In the meantime, Yong falls in love with Eun-chae, daughter of nobleman Byeon Shik and Si-hoo's stepsister, who can't forget her first love Lee Gyeom.