Once every one hundred years, the mermaids of the Hidden Lagoon and the witches of Sinkville get together for a party! The Top Witch greets the Mermaid Queen, who is wheeled through the streets of Sinkville in a bathtub. Mermaids Beattie, Mimi and Zelda can’t wait to meet a real witch, but when Tiga is unexpectedly sucked down the plughole into the Hidden Lagoon before the celebrations even start, things start to go magically, marvellously, megalodon-sharkally WRONG.