Patricius, a young man of Britannia, is taken from his home and family when Gaelic pirates attack his village. On his arrival in Ireland, he is sold as a slave to the cruel underking of the Dalriada tribe in the north. Six years later, Patricius manages to escape. His journey takes him through France to Ravenna in Italy. His subsequent plans to return to Britannia are side-tracked when he finds himself accompanying several monks to the island monastery on Lerinus. His devotion to his faith, honed during his captivity, grows as he studies with the monks.
Haunted by visions of the Gaels begging him to return to Ireland and share the word of God with them, Patricius gains support from Rome and his friends to return to the land of his captivity. His arrival is bitterly opposed by the druids, who have held power over the Irish kings for many years, and he and his companions must combat the druids to succeed in their God-given mission.