Based on a fictional book about a Robin Hood in Korea,Hong Gil Dong  (http://en.wikipedia.org/wiki/Hong_Gildong), this drama is very loosely based off of that book and the time it was set in.
Hong Gil Dong is the bastard son of a minister. He spends most of his time lazing the day away and taking goods from the various merchants. However, a local monk think he's destined to be more. Hong Gil Dong, for his part, really dislikes the injustices in this ranked world.
Heo Yi Nok has returned to Korea, fresh from China. She's an upbeat, naive and carefree girl. She's been living with her grandfather for the last few years. They both are merchants looking to make a fortune back in Korea.
Lee Chang Hwi, however, has been told since he was young that he's the rightful king of Korea. He bumps into Heo Yi Nok. This initially causes misunderstanding between them, but over time his heart is softening towards her.
Meanwhile Heo Yi Nok and Hong Gil Dong are developing a friendship that feels like (at least to Yi Nok) that it could be more.
As the question of rightful succession comes into play, Hong Gil Dong and Yi Nok are forced to take sides.