When Jonáš, a water sprite, and Karolínka, the miller's daughter, fall in love, they don't have it easy. Jonáš's father, the grumpy water sprite Hastrbuch, stands in the way of his love, while Karolínka has no idea that her beloved Jonáš is not human. Luckily, there is a young man named Libor who will do anything to protect the whole village, and especially Karolínka, from the water sprite villain. So what if he has to scheme and lie? The main thing is that he can get Karolínka to reject Jonáš and choose him, the brave Libor, instead.