Miranda the mermaid (Glynis Johns) is every bit the siren you'd imagine a mermaid to be. She meets Paul (Griffith Jones) while he's on holiday without his wife. Miranda hooks herself onto Paul's fishing line and drags him down into the water to keep him prisoner. She agrees to let him go if he takes her home to London with him. With no other option, he agrees, but he has to figure out how to disguise the mermaid so no one will know what's going on. Once they arrive in London, a series of events ensue that threaten to expose Miranda and Paul's secret.