The story begins in the realm of Queen Unda, mistress of the under-seas, surrounded by her nymphs, sylphs and mermaids, who disport themselves on the sands and in the waters of the deep. Berthelda, daughter of a fisherman and his devoted wife, has been stolen by the mermaids one day when the child is playing on the sands. Queen Unda rules that little Berthelda shall be left to roam in the Enchanted Forest, because her parents have taken fishes from the ocean, greatly to the annoyance of Unda and Neptune. Undine's mother has committed sin with a mortal and to atone for this her little baby, Undine, is taken to the shore near the fisherman's cottage, to be discovered by the fisherman and his wife. It is Undine's mission on earth to marry a mortal, and thus atone for the sins committed by her mother in loving a handsome young huntsman, whose untimely death likewise robs Undine's mother of her own life.