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.

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.