Mayre Griffiths, nicknamed Trot, is a small girl living on the coast of California, her close friend and companion is Cap'n Bill, formerly her father's employer and now a retired seaman with a wooden leg. As they walk along the beach one day, Trot wishes that she could see a mermaid, but Cap'n Bill cautions her that no man has ever lived to tell the tale after seeing a mermaid. Trot questions this, since if no one lived to tell the tale Cap'n Bill logically would never have heard it, but he doesn't have a good answer for her. Nearby mermaids overhear them, and the next day when the two of them are boating and row into a sea cave, grant Trot her wish. The mermaids explain that they are "sea fairies," and offer the girl a chance to experience their world. Trot is eager to go; Bill is not, but is too loyal to let Trot go by herself. Magically transformed into mermaid and merman, the two embark on an underwater adventure.
The two see amazing sights in the domain of King Anko and Queen Aquareine including an octopus who is mortified to learn that he is the symbol of the Standard Oil Company. They also encounter a villain called Zog the Magician, a monstrous hybrid of man, animal, and fish. Zog and his sea devils capture Trot and Cap'n Bill and hold them prisoner. The two protagonists discover that many sailors who are thought to have drowned have actually been transformed into zombie-like gilled slaves by Zog. Although protected by magic, Zog wishes to find a way to kill Cap'n Bill and Trot, only to spite his enemy who they have recently befriended, King Anko the sea serpent.
Tropes present in this work:
- Arch-Enemy: Zog and King Anko have a rivalry going back millennia.
- Atlantis Is Boring: The addition of Zog does a lot to spice the story up a bit, but the adventures before he shows up are mostly Filler, Wacky Wayside Tribe fare, mainly due to the lack of many adventures one can have underwater, which may explain why the book didn't sell terribly well, especially compared to the Oz books.
- Big Damn Heroes: King Anko arrives just as it looks like Zog really will kill Trot and Cap'n Bill, and squishes him to a pulp.
- Cheerful Child: Trot is generally quite cheerful and balances out Cap'n Bill's overly cautious pessimism.
- Constantly Curious: Trot is very curious and thoughtful and tends to ask a lot of questions, perhaps because she has the well-traveled Cap'n Bill as a ready Mr. Exposition, who gives her the opportunity to play The Watson on more than one occasion.
- Cool Old Guy: Cap'n Bill, who is said to have had many adventures before this, such as being stranded on a deserted island and Captured by Cannibals.
- Expy: Cap'n Bill is an expy of Naboth Perkins from Baum's earlier Sam Steele books. Of course, Cap'n Bill, thanks to his role in a much more popular book series, is much more widely known than Naboth Perkins.
- Fireball Eyeballs: Zog has these, despite being underwater. They get brighter when he's angry.
- Funetik Aksent: Cap'n Bill certainly talks like an old sailor — minus the swearing, that is. If you've watched any Popeye cartoons you get a sense of how Cap'n Bill speaks.
- Handicapped Badass: Cap'n Bill's wooden leg might have forced him to retire from the sea, but it doesn't stop him from going on all kinds of magical adventures. It does, however, give him some genuine trouble; with a wooden leg he can't run fast and has problems with uneven ground. The badass part is that he refuses to let this stop him. When transformed into a merman though he has a fully functional fin and is no longer handicapped in any way.
- Intergenerational Friendship: With Cap'n Bill, who acts as a father-figure to her.
- Meaningful Name: The mermaid queen, named "Aquareine", "Aqua", invokes the water in which a mermaid lives, while "reine" is similar to "Siren", also used as a word for mermaids.
- Mr. Exposition: Cap'n Bill has lived a long life and experienced much, and is always happy to answer Trot's many questions. Occasionally subverted, though, in that he often doesn't know any more than she does about the fairy-lands they end up visiting.
- Our Mermaids Are Different: Mermaids are a type of fairy, and overcome any logical realistic issues with their existence through being magic.
- Only Known by Their Nickname: She got the nickname "Trot" when she was a toddler learning to walk, because she took so many busy little steps. She grew to prefer her nickname to her real one, so her real name, Mayre, is only mentioned in passing a couple of times. Otherwise both narrative and characters refer to her as "Trot" or sometimes "Tiny Trot."
- Parental Abandonment: A strange version, possibly even an inversion, as both Trot's parents are alive and well, but simply fade out and are never mentioned again after she moves to Oz.
- Parental Substitute: Cap'n Bill is one to Trot, even though both her parents are alive.
- Mix-and-Match Critters: Zog is a dreadful creature, part fish, part man, part beast, part fowl, and part serpent.
- Seadog Peg Leg: Cap'n Bill and his wooden leg.
- Sea Monster: King Anko is a rare friendly sea serpent.
- Sirens Are Mermaids: Cap'n Bill at least is under this impression at the beginning of the novel when he tells Trot about how they lead sailors to their doom with their beauty, but the mermaids later disprove this.
- Super Not-Drowning Skills: The mermaids have this and grant it to Trot and Cap'n Bill; their magic creates a thin layer of air around their bodies so that the water never actually touches them, and they are still able to get oxygen out of the water. This magic also protects them from increased water pressure as they go deeper beneath the ocean. It even results in the two of them not even being wet when they come out of the water.
- Vile Villain, Saccharine Show: Zog is one of the most unambiguously evil characters in Baum's works; for about the first half the novel is a cutesy, carefree adventure under the sea, but takes a very dark turn suddenly when Zog appears.
- Wacky Wayside Tribe: The trope which Baum was so fond of using in the Oz series shows up here as well, with random encounters with types of fish and sea creatures. Up until Zog appears, it is almost like The Road to Oz but underwater.
- The Watson: Trot occasionally plays this to Cap'n Bill, or when anyone else has backstory they need to convey to the reader.
- Weirdness Magnet: Even before she gets to Oz, Trot has several weird and magical adventures quite by accident. This is hinted to be because she was marked by fairies at birth.