Simple story: Two children fall in a well and are captured by a nixie. She makes them work as slaves. While she is away, the children escape. As they are running from the nixie, they throw items behind them that magically change into obstacles blocking the nixie's way. A brush becomes a mountain full of spikes. A comb becomes a mountain with thousands of teeth. A mirror becomes a mountain with a slick surface. The nixie runs home to get an ax to chop through the mountain, only to find that the children have finally escaped.
The plight of the children has always reminded me of Hansel and Gretel. The kids get lost and then have to fight to find the way home. The Water Nixie is cute though and worth the read.