Based on the novels The Constant Princess (2005) and The King's Curse (2014) by Philippa Gregory, it is a sequel to the miniseries The White Queen and The White Princess. It centres around Catherine of Aragon (Charlotte Hope), the eponymous Spanish princess who became Queen of England as the first wife of King Henry VIII (Ruairi O'Connor).
Teenaged princess Catherine of Aragon, daughter of Spanish rulers Isabella and Ferdinand, finally travels to England, to meet her husband by proxy, Arthur, Prince of Wales, heir apparent of Henry VII of England, to whom she has been betrothed since she was a child. Unwelcomed by some, she and her diverse court, including her lady-in-waiting Lina, who is of Moorish ancestry, struggle to adapt to English customs. Catherine is horrified to learn that Arthur's younger brother, the arrogant Henry, Duke of York, is the author of the romantic correspondence she has received. When Arthur dies suddenly, her destiny as the one who will bring peace between Spain and England seems in doubt, until she sets her sights on Prince Henry.