It was a historical drama about a girl who infiltrates the palace in search of her adoptive mom s biological daughter. While there a princess who is set to be the crown prince’s new wife runs away because she’s in love with someone else. The girl stays behind and pretends to be her. Eventually she and the prince fall in love, and he finds out her secret, but they can never tell anyone or she will be put to death. I think the original princess eventually also comes back.