Physician Paul Marton (Griffith Jones) is drowning when an alluring mermaid named Miranda (Glynis Johns) suddenly rescues him in this fanciful drama. Afterward, Miranda insists that he take her for a visit to London -- which the doctor manages by disguising her in a wheelchair. While Miranda captures plenty of male attention there -- and arouses the suspicions of the doctor's wife (Googie Withers) -- she soon begins pining for her watery home.