When in Brittany, Ann goes for a solitary evening swim. She loses consciousness and is discovered by Jahaan, a Breton fisherman, living in the year AD 495. When Ann regains consciousness she has no memory of her previous existence.
Jahaan and his grandmother Norah care for her and she becomes part of their community. Later Jahaan and Ann marry but trouble surfaces when unknown to them Avel, a recreant priest, falls passionately in love with Ann.
When Ann and Jahaan visit the pagan City of Ys they find themselves caught up in a maddened crowd and witness the trial by 'swimming' of Gwendoline a young witch. Struggling through the crowd they are surrounded by soldiers and taken prisoner on the orders of Princess Dahut, the ruler of Ys, acting on information given her by Avel the priest. In prison, as they await their uncertain fate, they are unexpectedly helped to escape during a great storm.
What happens when Ann and Jahaan become separated brings the absorbing story to its end.