He sits on his jewelled throne on the Horn of Africa in the maps of the sixteenth century. He can see his whole empire reflected in a mirror outside his palace. He carries three crosses into battle and each cross is guarded by one hundred thousand men. He was with St Thomas in the third century when he set up a Christian church in India. He came like a thunderbolt out of the far East eight centuries later, to rescue the crusaders clinging on to Jerusalem. And he was still there when Portuguese explorers went looking for him in the fifteenth century.

He went by different names. The priest who was also a king was Ong Khan; he was Genghis Khan; he was Lebna Dengel. Above all, he was a Christian king who ruled a vast empire full of magical wonders: men with faces in their chests; men with huge, backward-facing feet; rivers and seas made of sand. His lands lay next to the earthly Paradise which had once been the Garden of Eden. He wrote letters to popes and princes. He promised salvation and hope to generations.

But it was noticeable that as men looked outward, exploring more of the natural world; as science replaced superstition and the age of miracles faded, Prester John was always elsewhere. He was beyond the Mountains of the Moon, at the edge of the earth, near the mouth of Hell.

Was he real? Did he ever exist? This book will take you on a journey of a lifetime, to worlds that might have been, but never were. It will take you, if you are brave enough, into the world of Prester John.