At opposite ends of Grandinsula, a remote pre-reformation Christian island, shepherds find a creature with strange footprints stealing their lambs, and fisherman find a swimmer near exhaustion struggling towards the shore. The child cannot stand, eat or speak like a human being; the swimmer says he is a prince in the unheard of land of Aclar, and declares himself to be an atheist. Severo, Cardinal and Prince of the island, is confronted by a double conundrum. Could an atheist be in good faith? Not if the knowledge of God is inborn; then the atheist must once have known God, and reneged on the knowledge. If he is a renegade from the truth, he must be burned as a heretic; but Severo would dearly like to save him. How could it be found out whether everyone has inborn knowledge of God, since the teaching of the Church as best known to the greatest scholar on the island is unclear? Perhaps by teaching the wolf-child to speak, and then asking her... That will take time. Meanwhile, it is worth while trying to demonstrate the truth of God to the mysterious atheist in argument.
What becomes of the argument, of the atheist, and of the wild child, and the effect of their fates on Severo, and the islanders who come in contact with either the Prince of Aclar, or the ferocious child Amara makes up the thread of the story. This is a fable about tolerance, and its conflict with moral certainty.