Johnny Baldwin works at the Mermaid Museum in an English seaside town, where he is bullied for being different. However, Mermaid Dessa adores Johnny, and he, in turn, is in love with her. He sees how she suffers being held against her will and promises to return her to her beloved ocean.
As the pair carry out a dangerous escape, the risk of being caught brings them closer. Dessa learns how vulnerable Johnny truly is, and when she sees what he is prepared to sacrifice for her, she is determined keep them together, impossible as this might seem.