The book opens with Catalina's childhood. Already betrothed to the Price of Wales, she grew up on the battlefield as her parents attempted to conquer Granada and therefore unite Spain under a Christian monarchy. The opening of this book recouts how the keys to the Alhambra were handed over to Ferdinand and Isabella. Flash forward many years, and Catalina is newly arrived in England to wed Prince Arthur. England is absolutely nothing like Spain: its cold, it rains, the language is foreign to her, and the customs and traditions are completely different. Henry VII is king of England, and scared of the treat of "pretenders-" that is, people who claim to be heirs to the throne. His fright leads him to be a merciless enemy to those who oppose him. Also prevalent at court is Margaret Beaufort, My Lady the King's Mother, who is the one in reality who runs England and the royal court.
Upon meeting her new husband, the impression is not favorable; but after a few months of being married, the couple fall in love. Arthur and Katherine spend every night (plus the one night a week they are supposed to be together) together, talking, and Katherine tells stories of her native land.
The story is very well known: Arthur dies of "the Sweat" (plague) after five months of marriage, and Katherine lives in poverty while the king and Spanish ambassadors haggle over what to do with her. Unbeknownst to them, however, Katherine made a pact with her husband on his death bed: to wed his younger brother, Prince Harry. With the determined conviction that she was meant to be Queen of England, Katherine spends the next seven years making plans; and does indeed marry Prince Harry, later King Henry VIII.
The Constant Princess was better than other Philippa Gregory books (such as The Virgin's Lover), but it definitely flagged in comparison to her better books. I had a difficult time understanding why Katherine was so stubborn in her need to be Queen of England; I got the impression that there was something deeper than just a deathbed promise. What was superfluous to the plot of the book were the last few pages of it, which details to a certain extent the divorce proceedings. I also think that the book could have been better copyedited.