The movie offers a fun showcase for Glynis Johns, who plays both a gymnastics instructor ("Caroline") and a mermaid ("Miranda"). "Caroline" inherits a house on the Cornish coast, while "Miranda," along with her playful companion "Berengaria" (Dora Bryan), resides in an underwater cave. The two women are related, share a striking resemblance, and when "Caroline" goes for a bike ride, her mermaid relative steps in—and …