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 In The Constant Princess, we were introduced to the character of Princess Catalina of Spain who through many tribulations finally became Katherine Queen of England. In The Other Boleyn Girl, we are introduced to two girls - actually three - who ruin everything for not only Katherine, but for the way the world worked as well. 
 Mary Boleyn is thirteen and married when the book begins with the beheading of someone that the king was close to. Mary is befuddled that the king didn't stop the beheading, thinking that he would have offered clemency. She asks her mother this, and her mother tells her that that is not how it works in the court and if she continues to think that way then she is a fool. A year later, and Mary's sister - Anne - has come back to the English court from France. By this time Mary is fourteen and has caught the wandering eye of the king. He flirts openly with her and the family begins to plot. Mary is commanded to woo the king and become his mistress, by this point Katherine is becoming infertile and the king is losing his patience with her. He must have a son to take over his throne. 
 After getting a girl on her, Mary does produce a son, but by this point it is already too late. While Mary was in confinement, her family commanded Anne to detour the king's affection, so that way his thoughts were always on Mary. But when she comes out of confinement, she can tell with just one look that the king is besotted with Anne and another plot by the family is formed. This time it goes for all the marbles.