This story is based off of an old Chinese legend and play called Liu Yi and the Dragon Princess. I found it in a collection of old Chinese tales, but there are versions all over the internet, too.
As the story goes, Liu Yi was traveling across the countryside when he came across a lovely but sad girl tending a flock of sheep. She asked him to deliver a message to her father, the Dragon King, because she was unhappy with her cruel husband. Liu Yi was going to be passing through the area where her father’s lake home was, so he agreed. When he arrived at the lake the girl had described, he announced his arrival and was led down under the lake to the home of the Dragon King. The king and his court were sad about the princess’s plight, but tried not to make too much noise about it. If the king’s brother, Qiantang, were to hear of this, his temper might cause great disasters like the ones for which he was currently chained in his dragon form.
Qiantang did catch wind of what was going on, however, and broke free from his chains in a rage. Seeing the huge dragon roar through the halls badly frightened Liu Yi. He was quite relieved when Qiantang returned quietly in human form during a celebration for the princess (who had arrived home, free, in the meantime). The dragon lord seemed more polite then, but when the Dragon King interrogated him, he admitted that he had killed 600,000 people and destroyed 800 square miles of crops in his revenge on the princess’s cruel husband. Qiantang then suggested that Liu Yi marry the princess, but the poor young man was so afraid of receiving the same fate as the girl’s previous husband that he declined, soon leaving for home.
As soon as he left the abode of the Dragon King, Liu Yi regretted his choice. He missed the sweet, beautiful dragon princess. It was too late, though, so he married first one human wife, then another, only to have each die childless. Finally, he took a third wife, who soon became pregnant. Over time, he noticed how much she reminded him of the Dragon King’s daughter. After their son was born, she revealed to him that she was in fact the princess, but had been afraid to say anything sooner. He was as overjoyed to finally be with her as she was to find out that he had always loved her. When she shared her long dragon life span with him, they lived together in the kingdom of the Dragon King for many happy years.