Medieval/Fantasy Plot Idea

Exactly 6 days before the prince's/princesses coronation to become ruler, they go missing. The kingdom is not notified and it is kept under wraps, the kingdom not really knowing what the prince/princess looks like is presented with a substitute. This substitute knows nothing of being a royal. They are then placed in a position of betrothal/ball/romance somehow. A prince or princess from another kingdom comes and knows nothing of the prince/princesses recent dissappearance. The substitute falls in love with newcoming prince/princess from other kingdom. But since they are not truly a royal it can't happen.
((Story moves on from there.))