In this History 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. He dealt the fatal blow to Richard III. We uncover what drove Rhys ap Thomas to betray not only his master but a King – and we reveal his remarkable story; from a childhood embroiled in the War of the Roses and exile to the continent, to a determined and ambitious man who brought an abrupt end to the Plantagenet line, carving the way for his own rise to power at the heart of the Tudor dynasty.