Yi Seoyo is a prince who grew up believing he was a Yang being. But when he reaches the age of thirteen, he discovers during a ceremony that he is merely a commoner, a fact that deprives him of his princely status and palace home. Just as he is about to forget about the palace’s splendor, his royal past, and Nareu-ye, the boy he promised to take as his mate if he were a Yang being, Nareu-ye appears in front of him with everything he has been stripped of. Then, in the distant future, Yun Seoyo is born, carrying the memories of his past life as Yi Seoyo. He meets Kwon Chaheon, who looks identical to Nareu-ye but has an entirely different personality. Despite trying to distance himself from Chaheon due to his painful past, he gradually finds himself opening his heart to him…