Original version

Once upon a time there was a sea mermaid who got lost and swam up the Wisła (Vistula) river. She took a rest on the riverbank. She looked around, fell in love with the harmonious surroundings and decided to stay.
Local fishermen started noticing that something unusual was disturbing the river’s calm waters and releasing fish from their nests. Not deliberating much, they decided to catch the damage-doer.
To their surprise, they saw an unusual woman whose legs were covered in scales, looking just like a fish tail, captured in the trap they’ve prepared. She asked them to release her, and the mermaid’s melodious voice made them fell in love with her. They apologized and let her swim freely. From that day on they were often gathering on the riverbank after a hard day of work, listening together to the mermaid’s soothing songs.
One day a rich travelling merchant found out about the mysterious creature and sneaked to the riverside in the evening. After listening to the mermaid, his greedy heart and soul desired to own her. Merchant’s mercenaries put up a trap and captured the mermaid, then locked her in a nearby hut and were waiting for the further orders.
The mermaid started crying and her cry was like the saddest song of the nature. People’s hearts were bleeding with sadness. A brave fisherman’s son couldn’t stand the torment brought upon the mermaid and gathered the locals. Together they defeated the guards and set the mermaid free.
“I will never forget your deed” said the mermaid. “I won't sing for you any longer, but whenever your people would meet overwhelming troubles, I will protect you".