Yizhimei was originally a character in the 24th chapter of the Chinese vernacular novel "Happy Enemies". As the "Happy Enemies" was introduced to Korea in the late Joseon Dynasty, the characters in the story began to appear in Korean literary works. When Li Qian (played by Lee Jun Ki) was 9 years old, he witnessed his father being killed and his mother and sister being imprisoned. He lost his memory because he could not bear the blow. Later, he became the adopted son of the Cheolseok family and was named Yonger. He gradually grew up to be a gangster. By chance, he recovered his memory and began to doubt his father's death. The only clue was the pattern on the sword that stabbed his father to death. In order to find the sword, Yonger became a thief and became a famous thief Yizhimei in the world. He specializes in punishing corrupt officials who are rich, powerful and have force, and fights against those in power and the corrupt social system. As a result, he became the most hated criminal of the court officials and the hope of the poor people. However, neither the people nor the court officials knew his true origins. By chance, Yizhimei meets Byeon Eun-chae (Han Hyo-joo), the daughter of aristocrat Byeon-sik (Lee Won-jong), and falls in love with her. Will Yizhimei be able to find out the truth about her father's murder?