When Nell’s parents succumb to the plague, she and her brother, George, fear what will become of them. As they follow the death cart to the graveyard, however, something miraculous happens. They cross paths with the king — who is struck by the fact that Nell looks nearly identical to his daughter, Princess Joan. In exchange for Nell’s promise to help protect the princess (by acting as her double), the king swears to take care of both Nell and George. A dream come true! The two become fast friends with the princess and many of the servants in the castle, and they start to think that their worries are behind them.
Some time later, to seal an alliance between their two kingdoms, the king comes up with a plan to marry off his daughter to the Prince of Castile. In a tragic turn of events, though, Princess Joan dies from the plague while traveling to meet her husband-to-be. Princess Joan’s brother, The Black Prince, decides that the alliance is far too important to let the marriage fall through — and he decides to force Nell to take the place of Princess Joan. It’s obvious, to Nell, that something is very wrong with The Black Prince, but she can’t quite put her finger on it.
When one of the king’s soldiers helps Nell and George to escape from the clutches of The Black Prince, they all try to make their way back to the king without falling prey to The Black Prince or the plague. But, can they make it back alive? And how will they ever explain it all to the king even if they do make it back?