Watch The Little Mermaid for Free: How to Stream the Movie Online at 123Movies. . This charming, lively tale based on Hans Christian Anderson's beloved children's story comes to splendid life in an enchanting animated film. The beautiful and adventurous mermaid, Princess Lena, lives with her family in a watery castle and has everything she could ever need, but she longs for only one thing; to explore the world of humans. Determined to marry Prince Stefan, whom she sees when a whirlpool takes her to the surface, Princess Lena must turn to the evil sea witch Cassandra and pay a heavy price for her promise of help.