The merfolk of Liri have lived in the waters off the coast of Denmark for centuries, until an archdeacon who sees their godless lives as a potential temptation casts them out with bell, book, and candle, destroying their town and causing unbearable pain to the merfolk until they flee the area. Only four are immune: Tauno, Eyjan, Kennin, and Yria, the half-human children of King Vanimen. The book follows the siblings, as well as the rest of their people, as they take separate paths in search of a new home.
The Merman's Children contains examples of:
- The Ageless: Merfolk age at the same speed as humans until they reach adulthood, and then stop. Undersea life is so hard and dangerous that many die young, including three of Vanimen and Agnete's children, but the lucky ones live for centuries.
- Angst Coma: Father Tomislav's late wife Sena was an orphan raised by relatives who mistreated her. During her marriage to Tomislav she had more miscarriages than live births, and their three oldest surviving children all rebelled and moved to distant lands. After the birth of her youngest child Nada, she withdrew into silence and spent ten years in bed, barely moving, until she finally died.
- Apparently Human Merfolk: The merfolk look human aside from their green hair, gills, and webbed feet.
- Identity Amnesia:
- The day Yria is baptized as Margrete, God grants her a new soul, causing her to forget everything about her life in the sea, including her old name. She only remembers things she learned on land — her new name, the tenets of the Christian faith, and the Danish language. The next time her siblings visit, she doesn't recognize them and shrinks from them in fear.
- Nada was a young woman who committed suicide by drowning herself in a lake. Afterwards she became a vilja, haunting the lake and remembering nothing of her former life. When she sees her former lover Mihajlo, she regains some memories of him, but Mihajlo flees in a panic, and she calls after him for a few seconds before forgetting him again and running off laughing.
- The Magic Goes Away: Faerie creatures of all kinds have been slowly dying out thanks to the rise of Christianity and Islam and the encroach of civilization into wild areas. One of the biggest problems the merfolk face on their journey is that many areas of the sea have been made uninhabitable by exorcisms. The exception is in one region of the Dalmatian coast, where people follow a form of Catholicism that is more tolerant of Faerie than the kind practiced in most of Europe, but even there, Pavle Subitj believes that Faerie will be extinct in a few centuries at most. By the end of the book, all the merfolk except Tauno have converted to Christianity and traded their agelessness for mortal souls. Most of them marry humans, and in a few generations their descendants will be indistinguishable from any other mortals.
- Mama's Baby, Papa's Maybe: The merfolk aren't monogamous, so most of them have no idea who their fathers are. Vanimen's children are the only exceptions, because their human mother Agnete followed human sexual mores even while living in Liri.
- Meaningful Rename:
- Yria has always been weak, and her older siblings worry she won't be able to survive a long journey. She takes after her mother enough to pass for human, so they leave her on land with Father Knud to be raised as a human girl. The archdeacon renames her Margrete, after a saint, and arranges for her to be baptized.
- In Dalmatia, all the merfolk take on new names when they convert to Christianity and gain mortal souls. Vanimen becomes Andrei, and Eyjan becomes Dagmar.
- Sex for Services: The selkie Hauau helps to repair the damaged cog Herning and navigate back to safety in exchange for sex with Hooker with a Heart of Gold Ingeborg.
- Shameless Fanservice Girl: Merfolk don't wear clothes except on special occasions. The siblings usually wear clothes on land, but despise the feeling and the smell and take them off at the first opportunity, including the oldest girl Eyjan, to the delight of the sailors on Herning.
- The Soulless: Merfolk have no souls. Unlike most examples, they're as capable of feeling love and other emotions and following moral rules as the humans are, but when they die, they just disappear instead of going to Heaven or Hell.
- Tears of Joy: Many of the merfolk cry when they are baptized, and receive a new soul from God.
- Translator Microbes: In Greenland, Panigpak the angakok gives Tauno and Eyjan an amulet which, when worn, gives them the ability to understand, speak, and think in any language. It also gives them the ability to understand concepts that their own language doesn't have words for, without anyone needing to explain those concepts to them.
- Underwater City: The inhabitants of Liri take advantage of the water's support to build delicate houses of sea plants and ivory that would collapse on land. One exception is Vanimen's hall, which is made of stone and coral.
- Underwater Ruins: The island city of Averorn was drowned long ago by a hungry kraken who was angry that the townspeople had stopped making their usual sacrifices. Tauno, Eyjan, and Kennin travel there in Herning to loot the ruins. Most buildings have completely collapsed, but a few structures are still partly standing, including a roofless house where a skeleton tries to shelter two others. The siblings are delighted to see burst-open vaults full of gold and jewels.
