The plague

Fifteen year-old Nell bears an uncanny resemblance to King Edward the Third's daughter, Princess Joan. The king brings Nell and her brother George from the murky streets of 14th-century London so that Nell can be the body double for the princess in times of danger. When the plague takes the princess' life, Joan's brother, the Black Prince, forces Nell to continue in her role so he can marry her to the Prince of Castille in Joan's place. Nell, however, is determined to return to England to report the princess' death to the King.