During the Joseon Dynasty, Ryung acted as a hooligan in the marketplace by day but at night he was a thief who robbed corrupted government officials to give to the poor.

Geom is the son of virtuous nobleman Lee Won Ho, a trustworthy supporter and brother to the king and a central member of the secret organization Cheonwoohoe, comprised of important nobles, with the king as their leader. He lives peacefully in a plum-tree-surrounded house with his wife, daughter, and son. However, a mysterious rumor is spread that he is plotting to overthrow the king. Knowing about this, the king kills Lee Won Ho as he believes that 'two suns cannot exist in the same sky'. Geom witnesses the incident from the inside of a safe where his father hid him. Later, he is saved and adopted by a retired thief, Seo Dol, after he becomes stricken with amnesia from the shock of losing his father and being forced to throw a rock at his mother's head to prove that he wasn't her son. 13 years later, Geom--now renamed Ryung by Seo Dol--starts to regain his memory and begins to search for his older sister Yeon--only to see her being sentenced and hanged. Geom seeks revenge; with the emblem on the killer's sword he remembers from 13 yeas ago his only clue, he swears that he will find the one who killed his father and murdered his older sister. To find the sword and its owner, he disguises himself as Iljimae and breaks into the nobles' estates. During the course of his actions, he winds up fighting not only for himself, but also for the common people, although at the core his desire to bring the person who ruined his family to justice burns bright.—Colleen16

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-i". Thirteen years later, Yong-i 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-i 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.