Angry, Princess Nestis stomps with her tail fin. Her little sister Undine, affectionately called "Sea Barnacle" by her, has been captured by humans, and neither the Sea King nor the Council of Lords dares to rescue her. But Nestis is not afraid, even of the most terrifying sea monsters. Together with her friends, she sets off on a rescue mission, and it turns out that clumsy giant squids and calculating electric eels make excellent allies, not just in the fight against sneering sharks and whimpering water spirits. However, the biggest problem must be solved on land: because fish have no legs. Petra Hartmann takes her readers into a slightly different underwater world filled with humor and attention to detail. Despite the fantastic mermaid theme, readers will also find many nature-related observations from the North and Baltic Seas and get to know the sea creatures.