Catalina of Aragon is betrothed to Prince Arthur Tudor, the crown prince of England, after a long negotiation between his father, King Henry VII, and her parents, King Ferdinand II of Aragon and Queen Isabella I of Castille. Upon her arrival in England, Arthur is initially coldly polite and their marriage is unbearable for Catalina, but they secretly begin to love each other. However, Arthur succumbs to the sweating sickness three months after their wedding and, on his deathbed, makes Catalina swear that she will deny they had consummated their marriage so she can still be fit to marry his brother, Prince Harry and become queen in order to carry out their plans for the country. Arthur's mother, Queen Elizabeth, dies a year later. King Henry, lusting after Catalina, ignores his mother Margaret Beaufort's advice of the political disadvantages of him marrying Catalina against the advantages of marrying Catalina and Harry, and proposes to Catalina. Catalina initially accepts, but when she realizes she will have no role as queen except to bear children, she declines and pressures Henry to marry her to Harry.
Henry betrothes the two, but most of the English court begin to neglect and mistreat Catalina, who can barely afford to keep her own household; while her parents demand the English crown pay for Catalina's allowance as the Princess of Wales, Henry refuses to sponsor her until her parents pay the remaining half of her dowry, and neither will give her money. Henry stalls their wedding by claiming Harry is too young, but after Isabella's death, Catalina hears rumors that Henry had set aside her betrothal years ago and is planning to marry his family to her mad sister Joanna and her children, the heirs to Castille. Ferdinand sends the other half of the dowry, but when he hears of this, he commands the Spanish ambassador to return the dowry safely but makes no mention of his daughter's fate. Catalina is in despair, but her luck changes when Henry dies of sickness and Harry disregards his father's warning and marries Catalina for love. She manipulates Harry so Margaret loses her power to intervene, and after Margaret's death, they are crowned King Henry and Queen Katherine.
Complications arise in Catalina's first pregnancy, and eventually she accepts that the child was miscarried. Upon her return to court, she learns of a story of a scandal between one of her maids-in-waiting, Anne Stafford, and one of Harry's friend. However, she soon realizes that the story is a cover-up for Harry having an affair with Anne, who has become his mistress during Catalina's absence. Harry suspects Catalina and Arthur consummated their marriage, since Anne (a virgin) acted differently from Catalina when they consummated their marriage. Catalina manages to fool him and they reconcile. Their second child, Henry, is made Duke of Cornwall, but he dies less than two months later, which estranges their marriage. She is eventually made Spanish Ambassador and bridges Harry and her father to invade France together. During his absence, the Scots declare war on England, and Catalina, as regent, successfully leads the English army to victory. She sends Harry the seal of the Scottish king as well as a cryptic message hinting at another pregnancy.
In the end, it is revealed that of all of Catalina's children, only Princess Mary survived. Harry went on to have more mistresses, all of them she thought he would eventually be bored with, but his latest mistress, Anne Boleyn, is the most ambitious and is trying to take her spot as queen. Catalina vows to keep her promise to Arthur and proudly decides to fight for her right as queen.