Henry IV (1984)

During a costume party a wealthy young man falls off his horse and lands on his head, after which he believes that he is who he's dressed up as: 11th century Holy Roman Emperor Henry IV. Twenty years later those who once called themselves his friends, with doctor in tow, return to his castle - where the servants continue the medieval costume charade and refer to the main character as "Your Highness" - in order to try to try to cure him of his insanity.