Season 1 of The Duo premiered on February 7, 2011.

Two children are born in the same village at the same time. One child is born as the son of the wealthy Lord Kim's family, while the other child, not even knowing who his father is, is born in a mud hut as the son of a poor woman named Maksun. However, the mother of Lord Kim's family ends up dying after giving birth to the child. To have the child breastfed, Lord Kim brings Maksun, who also gave birth on the same day. For her son's well-being, Maksun secretly switches the child with her own. Thus, Maksun's son gets raised by Lord Kim's family with the name Gwidong, and the real child of Lord Kim's family grows up with the name Chundoong. However, thanks to a twist of fate, Gwidong and Chundoong become friends and end up loving the same girl named Dongnyeo.