Thora - half-mermaid, half-human - sails to London with her mermaid mother, Cosmo the peacock, and her newest friend, Shirley the sea horse.

While in London, Thora encounters the aging actress and kleptomaniac, Pamela P. Poutine - a mermaid Cruella de Vil. Pamela will steal anything she can - including Shirley, who's a rare and priceless green dragon sea horse. And when Pamela finds out that Thora has the legendary projectionist's ring, she'll stop at nothing to get her hands on it ...