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...