While on holiday at the seashore, siblings Francis, Mavis, Bernard, and Kathleen learn that a mermaid has been captured and is being put on display at a nearby carnival. The children decide to rescue her. They are helped by Reuben, a boy who's been raised by gypsies. As a reward, the mermaid, Freia, takes them all to visit the Merkingdom.
Wet Magic contains examples of:
- "Be Quiet!" Nudge: Francis asks the Queen about the King, not knowing that the King was lost at sea and she doesn't like to be reminded of him. His guide, a mackerel, jerks his coattail and whispers, "Hush!"
- Caught Coming Home Late: As Francis and Mavis are sneaking home early in the morning after rescuing the mermaid, they're caught by Mrs. Pearce, who owns the house where the family is staying. She assumes they went shrimping, and they let her believe it. She agrees not to tell their parents as long as they don't do it again. That morning, Bernard and Kathleen have a wonderful time at the beach while Francis and Mavis are asleep, and Mrs. Pearce tells them they got what they deserved by missing out on the fun.
- Celebrating the Heroes: Freia's mother is the mermaid queen, who throws a banquet in honor of the children who saved her daughter's life.
- Crappy Carnival: The carnival where the mermaid is on display is located on an ugly, litter-strewn piece of wasteland. It has no stalls or booths and, before the circus tent is opened, no attractions besides various rigged games. The children play a game that involves throwing hoops over prizes to claim them, but every time they win, the people running the game change the rules so it doesn't count.
- Disguised in Drag: Reuben runs away from the gypsy camp to look for his biological parents. Mavis brings him one of her outfits as a disguise. He doesn't have to wear it very long before Freia arrives to take the children to her kingdom, where they all wear seaweed clothes.
- Domed Hometown: The Merkingdom is full of a substance that is neither air nor water, that both fish and humans can breathe and that sea creatures can swim around in. It's protected from the ocean by a giant dome.
- Forever War: The Mer-people have been warring on and off with the Under Folk, who live in the deep sea, for 3,579,308 years.
- Human Pet: After Kathleen forgets who she is, the Queen of the Under Folk adopts her as a pet, names her Fido, and has a collar made with her name on it.
- Honorary Aunt: The children's Aunt Enid is actually their grandmother's friend. They dislike her because she's much stricter than any of their real aunts.
- Identity Amnesia: The Under Folk capture the four siblings and give them all a drink that will cause them to forget who they are. Three of them take the antidote they have, but Kathleen doesn't because she wants to forget her guilt over breaking the sky. It turns out the King and Queen of the Under Folk are a human couple, and the Professor of Conchology is the vanished King of the Merkingdom. All were given amnesia.
- Laser-Guided Amnesia: When the humans leave the Merkingdom, they're all given a bottle to drink from that will erase their memories of their adventures.
- Lighting Bug: The merfolk use phosphorescent fish as torches.
- Luke, You Are My Father: Reuben is told that after he leaves the Merkingdom, the first two adults he meets will be his biological parents. A few minutes after he arrives back in England, he meets the King and Queen of the Under Folk, whose memories of their human lives have been restored, and whose memories of their time under the sea have been erased. Reuben shows them the little shirt and shoe that he's had since he was a baby.
- Nice Job Breaking It, Hero: The children are under strict orders not to touch the sky dome, which protects the Merkingdom from the Under Folk. The dome is indestructible from the outside but can be destroyed with a touch from the inside. When the children climb to the edge of the dome, Kathleen can't resist the urge to touch it, and the sea comes pouring in. A violent battle with the Under Folk ensues. At least the children are protected from drowning by the locks of mermaid hair around their necks.
- Refugee from TV Land: The tunnel that leads into the Merkingdom is full of magic books whose characters can be brought to life by opening the book to the right page. The Under Folk send Bookworms in to let unpleasant characters out, and the Merkingdom is attacked by a horde that includes both fictional villains and the protagonists of books like Eric, or Little by Little and Elsie Or Like A Little Candle. The children let the heroes out of the books to fight them.
- Sand In My Eyes: Reuben rubs his eyes as he talks about the death of his adopted mother. Mavis says "Don't cry," but Reuben says he just has a cold in his head.
- Silly Reason for War: The children eventually learn what started the war: an Under-man accidentally trod on the tail of a sleeping merman and didn't apologise because he was under a vow of silence. Rather than wait for an explanation, the Mer-kingdom declared war.
- Sneaking Out at Night: Francis and Mavis sneak out after bedtime with a wheelbarrow to rescue the mermaid. Following Bernard's advice, they tie wads of towels and stockings to the tire of its wheel to stop it from making noise.
- Year Inside, Hour Outside: The siblings are worried that their parents will miss them if they spend too long in the Merkingdom, but as Princess Maia explains, the first principle of magic is that time spent in other worlds doesn't count in your own. The children spend days with the merfolk, but when they return to their world, no time has passed.
