They needed a prison for the most dangerous man in the World. Napoleon had seized supreme power in France. He’d marched his armies from Portugal to Moscow. But now he was a prisoner. His captors needed a prison from which escape was unthinkable. Their answer lay in the South Atlantic. A scrap of land a thousand miles from the African coast. St Helena. Dan Snow visits this remote island to learn more about its history. From accommodating some of the British Empire's greatest enemies to its role during - and in the aftermath of - the transatlantic slave trade.