This is a story of conspiracy and betrayal, of a lust for power and a lost allegiance; the story of the man who killed King Richard III. In this documentary we set out to prove that the Welshman Sir Rhys ap Thomas, master of Carew Castle in Pembrokeshire, killed King Richard III, changing the course of British history. Sir Rhys ap Thomas had sworn allegiance to King Richard III. He had accumulated lands and status in Wales that were dependent, in part, on his loyalty to Richard. But at the Battle of Bosworth he betrayed him, fighting on the side of Henry Tudor.