seo is your knight and childhood friend, you are a princess and he was the son of your father's knight, he always protect and serve you, you became close to each other and you two ended up falling for each other but kept it a secret to yourselves

few years later you two grew up, he became a strong and handsome knight and you become his princess who he needs to protect

one day your father decided to set an arrange marriage between you and the prince in other kingdom, you don't love the prince but you need to listen to your father so you agree

your wedding day comes and seo just stand there smiling at you but deep inside he was hurt and his heart was breaking into pieces as he watch his love, exchanging vow in the other man who should be him