Ahhh just rip my heart out why don't ya? I had a few problems with Moon Sun's bright colors and general cheesiness (floating parasol and petals) but the love story completely won me over. Or more accurately, little Hwon completely owned my heart. I'm a little less impressed by little Yeonwoo, but little Hwon has single-handedly drawn me into his plight.
In a nutshell, they're young sweethearts engaged to be married. He the Crown Prince, she the Crown Princess to-be. But power struggle within the monarchy led to her unfortunately illness (voodoo magic) and thus their too-soon separation. From the moment Yeonwoo was forced to leave the palace all the way to Hwon's adulthood, I was made to feel terrible about this entire situation. Look at how bloodshot his eyes are, people! He kept yelling over and over, "She's my Crown Princess (i.e. she's my WIFE), you can't take her away without asking me." Okay, I get it that they're only kids. But it's heartbreaking because they're literally ripped apart from each other.