The story is an adaptation of the Hans Christian Andersen fairy tale.
Once upon a time, far over the sea … Fozzel, King of Faber, finds that his island kingdom is running out of cockles (the official currency). He decides to marry off his daughter, Princess Clarissa, to Prince William, the son of his old rival, King David of Cardonia. After the wedding, he intends to dispose of the Prince and claim the inheritance.
Meanwhile, deep down below, in the Mer Kingdom, Princess Miranda, mermaid daughter of the Sea King, has come of age and is preparing to make her first ‘Ascension’ to the surface. It is a joyful time and all are there, even Wallum the Whale has paid a visit. The ceremony is interrupted by sailors above, who are throwing their garbage into the sea. The garbage descends upon the Mer Folk. Miranda, known for her beautiful singing voice, resolves to use her Ascension to go to the King of the Humans and sing about how beautiful the Mer World is. This, she feels, will shame the humans into not polluting the seas. Her constant companion, Acme the sea otter, is keen to accompany her – but a mermaid’s first Ascension must be made solo.
She ascends to the surface and her first glimpse of the world above captures her heart – but not as much as her first glimpse of Prince William, who is sailing to Faber, dejectedly, for his unfortunate wedding. William sees her in the distance. Meanwhile, Wallum, who is on his way home, is struck in the eye by rocks and garbage being thrown overboard. He loses his way and crashes into the ship, knocking William into the sea. Miranda rescues William and sets him, unconscious, on the shore.
She swims to the shark-guarded cave of the Sea Witch to ask for legs. The Witch has a potion that will give her legs … but take away her voice for a year, unless she is kissed by a prince – in which case her voice will return and her legs will remain forever. Miranda does not know, of course, that William is a prince.
Back on the surface, Miranda is washed up on the shore of Faber, unconscious, where she is found and brought to the castle. She awakens to see the prince and also to overhear the plot against him. But she cannot speak. Acme has been shadowing Miranda all this time and dashes off below to tell Miranda’s mermaid friends what has happened. Two of them go to the Witch, who takes their beautiful hair to prepare a potion.
The hairless mermaids arrive just as the wedding party has assembled. Miranda drinks the potion and has momentary use of her speech. She reveals the plot against William. Fozzel and Clarissa make a break for it, the Prince recognises and kisses Miranda … and, of course, they marry … to live happily ever after.