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. Sibeal Pounder brings together the characters from her brilliant, bonkers, bestselling Bad Mermaids and Witch Wars series for a once-in-a-lifetime World Book Day adventure.