Disclaimer: Any characters you recognise do not belong to me, and likewise nor do the historical figures or recognisable science-y things like the TARDIS. Everything else however comes from my head. The characters of Catalina/Catherine and Arthur are based on "The Constant Princess" by Philippa Gregory.
Full Summary: Catalina was always destined to marry Arthur, the future King of England, not his brother, soon to become Henry VIII. Arthur's death changed history, leading England to the King that would make them leave the Catholic Church. But when one lonely Catholic, the last of his kind, wants more people to understand his faith, he reaches back to the 1500s, fighting to prevent Arthur's death, the Doctor must step in to stop the course of history changing. Forever.
Prologue
The young couple curled together in front of the fire, their hands entwined. They were alone in Catalina's chambers as they had been many times before, unbeknownst to the other inhabitants of the castle. The fire was flickering, close to going out. Catalina stretched her legs, moving her feet close to the flames to grasp the final bit of heat from the dying light. Her husband shifted, supporting her gently.
He changed his mind and gathered her close to him, folding his arms around her as though he couldn't bear to let her go – which indeed he couldn't. In her turn, she twisted so her lips touched his neck, kissing it softly. He could barely feel her touch. Without talking, they lay back together on the cushions surrounding them, as though they could read each other's thoughts.
Dark brown hair shimmered around her shoulders, matching the eyes that glinted, warm but steely. He was her opposite, with light blonde hair, bright blue eyes and the Tudor smile. A match made to empower England and make them allies to the powerful Catholic Spain, had ended in a match built on mutual respect that turned into love and understanding.
Of course, this was no ordinary couple. This was Catalina de Aragon, Infanta of Spain, and Arthur Tudor, Prince of Wales. Arthur's death would change the future of England forever.
So would his survival.
A/N: Just in case anyone interprets the summary differently to how I mean it, this story is not anti-Catholic in any way, it's just an exploration of what might have happened if Arthur had lived and England had never broken with Rome etc. I just wanted to get that out of the way. Just in case!