Sanna, a sixteen-year-old mermaid/human hybrid who has grown up in the sea, wants to search for her landish mother. The witch Sjældent, who has spent the last two years teaching Sanna magic, casts a spell to turn her into a human girl and leaves her on the shores of the Thirty-Seven Dark Islands, a small, rocky archipelago in the northern Atlantic, with instructions to find something precious beyond imagination and bring it back to Sjældent. Sanna is taken into the castle of Baroness Thyrla, a powerful witch who wants to use Sanna for her own purposes.
Mermaid Moon contains examples of:
- Arranged Marriage: As soon as Sanna proves to Thyrla that she has magic powers, Thyrla betroths her to her seventeen-year-old son Peder so they can have powerful grandchildren Thyrla can steal the life from or manipulate for other purposes.
- Back from the Dead: Sanna, who is a more powerful witch than either Thyrla or Sjældent, is able to revive Peder and most of the drowned townspeople. But all of them are left with a disability of some kind - Peder loses his voice, Kett goes blind, and others become deaf, crippled, or unable to feel pain.
- Broken Pedestal: The "treasure" Sjældent wants is actually Peder. She wants to steal his life, and unlike Thyrla, she's powerful enough to steal life from a corpse. When Bjarl and Sanna try to stop her, she freezes Bjarl and blows flames at Sanna. Sanna turns the flames back on her, burning her to death.
- Chosen Conception Partner: Thyrla has had a number of children fathered by various sailors, born in secret, and killed in infancy to keep Thyrla young. Eventually she decided that a living child might be of use to her, so she married a weak, passive man named Lot who wouldn't get in her way. Once she was pregnant with Peder, she killed Lot too. Few people noticed his absence.
- CPR: Clean, Pretty, Reliable: Averted. After Peder is stung to death, Thyrla pinches his nose shut and breathes into his mouth until she's exhausted, but fails to revive him.
- Dem Bones: Thyrla has prolonged her life by many years by stealing other people's remaining lifespans, including her mother, her uncle, Lot, and all of her children besides Peder. She keeps the bones of her victims in her bedroom, where they can speak and have limited movement. They act as her advisors.
- Fiery Salamander: Sanna asks Thyrla for a demonstration of her magic. Thyrla coughs a fire-breathing lizard out of her mouth. A minute later, she kills it with her shoe.
- Gilded Cage: Thyrla keeps Sanna in a lavish room that is warded to prevent her or any of her magic from escaping.
- Half-Human Hybrid: When she lived in the ocean, Sanna's human ancestry made her different from the others in many ways - she was weaker and slower, her tail was shorter, her hair never grew past her waist, and her eyes were so sensitive that she had to close them in sandy waters.
- Harmless Freezing: Bjarl is frozen solid for a few minutes, but after Sanna kills the witch responsible, he unfreezes with no ill effects.
- Hookers and Blow: Peder's only interests are women and wine (the hardest drug available on the Thirty-Seven Islands). Thyrla has encouraged his interest in shallow hedonism while discouraging him from anything intellectual or practical so he will never leave her control.
- How Dad Met Mom: Sjældent tells Sanna the story of how her father, Bjarl, met her mother, Lisabet. Lisabet was washing clothes for her family where a river ran into the sea. Bjarl was swimming near the shore, looking for landish fruits to eat, when he heard Lisabet crying because she didn't want to wash clothes. Bjarl swam closer and looked up at her from under the surface. Lisabet looked down and was so startled she fell into the water, where Bjarl caught her.
- Immortality Seeker: Thyrla is at least a few centuries old. She wants to extend her life indefinitely by stealing other people's lives.
- Ladykiller in Love: After seducing countless peasant girls, Peder falls in love for the first time with Sanna. Unfortunately, Sanna is completely uninterested in him and spends most of the book trying to find a way to avoid marrying him.
- Laser-Guided Amnesia: The night of Sanna's birth, Sjældent cast a spell to erase everyone's memory of the event. None of the other merfolk remember anything about Lisabet, not even Bjarl. The spell also affected the landish people, so that Lisabet has no memory of falling in love with a merman and bearing his child, and her family doesn't remember she was ever pregnant. Only Sjældent remembers Lisabet's name and the story of her romance with Bjarl, but even she doesn't remember where Lisabet lived.
- Long-Lived: Merfolk live more than twice as long as landish folk.
- Magical Eye: Thyrla's right eye has two irises and two pupils. One can see the past, and the other the future. Discomfort in her eye can also warn her of things to come. Around Sanna, Thyrla's eye itches unbearably, which means the unknown. She keeps it covered with an eye patch so the townspeople don't know there's anything special about it.
- Maligned Mixed Marriage: Love between a human and a mermaid is heavily frowned upon by both species. Sjældent worked her forgetting magic to protect both Sanna and Bjarl from the cruel treatment they would face at the hands of many of their fellow merfolk if it was known that Sanna had a human mother.
- Meaningful Name: Thyrla named her son Peder, meaning rock, because the rocky island on which their castle was built seemed like more of a father to him than Lot. She thinks that he's still as foolish as a rock.
- Mermaid Problem: Merfolk's genitals are on the front of their tails, hidden by a belly fin. Also inverted, as Sjældent explains to Sanna how sex between humans works.
- Missing Mom: As a child, Sanna was the only girl she knew to be raised by her father rather than by her mother or another mermaid. When she asked Bjarl where her mother was, the only answer she got was "Not here." She didn't learn her mother was a human until she was fourteen, when Sjældent decided to let her know.
- One-Steve Limit: Averted. When Sanna asks Thyrla if she knows anyone named Lisabet, Thyrla lists off four women by that name. It's also mentioned that there are several Eriks and Knuds.
- Scary Stinging Swarm: Early in the book, Sanna gives a simple demonstration of her magic for Thyrla, but she accidentally attracts the attention of some bees in the courtyard, causing them to fly in and attack Thyrla and Peder. During Sanna's wedding to Peder, she tries to escape the marriage by causing all the petals to fall off the courtyard roses and fly into the air, but in doing so she also enchants the bees, and her Power Incontinence causes them to sting Peder to death.
- Screaming Birth: In the first chapter, Lisabet rushes to the seashore to give birth to Sanna. She tries hard not to make a sound so the townspeople won't find out she's having a merman's child, but when labor begins in earnest, she can't hold back her screams.
- Sirens Are Mermaids: Mermaids use their magical voices to force sailors to jump off their ships. Sanna thinks this is a horrible practice, even though her Aunt Shusha has told her that sailors rape and murder mermaids.
- Wedding Smashers: Unbeknownst to Sanna, the other young mermaids are planning to use their song to kill the landish people, partly in a misguided effort to save Sanna and partly just to show off their voices. They carry out their plan right after Peder is killed by bees, as all the wedding guests flee the courtyard in a panic and try to escape to the other islands. When they hear the mermaids' song, most of them run into the ocean. Dozens die before the older merfolk arrive and put a stop to it.
