In the Norse town where he ends up, events transpire which lead him to accompanying several warriors on a quest into Jotunheim, the land of the Trolls.
Nancy Farmer later wrote two sequels, Land of the Silver Apples and The Islands of the Blessed.
The Sea of Trolls and its sequels provide examples of the following tropes:
- Beauty Equals Goodness: Specifically subverted with Pega and Thorgil, as Jack notes that even though Pega is hideous she has a good heart and beautiful voice, and Thorgil's bad temper negates her natural beauty.
- Belligerent Sexual Tension: Thorgil and Jack, by the third book.
- The Berserker: Lots of literal berserkers (actual Vikings) appear in these books.
- Big Ol' Unibrow: Olaf One-Brow, Jack's master in the first book, is named as such because he has one brow.
- Broke Your Arm Punching Out Cthulhu: In the second book, Thorgil punches Satan in the face, but her hand becomes paralyzed because of it.
- Confusion Fu / Insanity Immunity: Berserkers are immune to trolls' mind-reading powers because their mindless, frenzied style of fighting is impossible for the trolls to decipher.
- Disabled Love Interest/Disabled Snarker: Thorgil becomes these after punching Satan in the face, which leaves her right hand paralyzed.
- Eldritch Abomination: A few appear, including one who appears to be Satan. (At least, that's what a Christian would call him. People from other religions see him differently; for example, Thorgil sees him as the Hell Hound Garm.)
- Here There Were Dragons: In Jotunheim, however, they're still thriving.
- Lady Looks Like a Dude: Deconstructed right down to the bone with one character.
- The Low Middle Ages: The first book takes place circa 793 C.E., when the Vikings started raiding.
- Mentor Occupational Hazard: Seemingly played straight, then averted in a Twist Ending. The Bard was not dead. He was actually disguised as a bird all along.
- He actually does die in Book 3.
- Naked People Are Funny: Freya's cats naked and Frith naked. Also counts as Squick. Averted with Rune, whose nudity in his introduction is described tragically.
- Our Elves Are Different: They're of The Fair Folk variety. They're also way different from other fictional elves in that they look monstrous under all that glamour.
- Our Dragons Are Different: They live in cliffside caves and raise their young like birds. Their blood can even give you the ability to understand bird-language, as per the myth of Sigurd and Fafnir.
- Our Mermaids Are Different: Young merfolk look like the classic "beautiful-human-with-a-fish-tail" variety, but as they mature they turn into grotesque Fish People. They can only avoid this fate by marrying a human.
- Psychic Powers: Lots of characters have Psychic Powers, including trolls.
- Sacrificial Lion: Olaf in The Sea of Trolls, who is slain by a massive bear.
- Satan: He makes an appearance near the end of The Land of the Silver Apples.
- Speaks Fluent Animal: Halfway through Sea of Trolls, Thorgil swallows dragon blood, giving her the ability to speak bird.
- Spoiled Brat: Lucy, and it gets much, much worse in book two—to the point where everyone around her is convinced it's the result of demonic possession. The first thing she does in book two is try to get away with secretly wearing an iron necklace during a ritual meant to celebrate Winter Solstice despite it being forbidden. For this, she loses her role in the ceremony and then demands that her father buy her a slave—specifically, the girl who took her place during the ceremony.
- Why Did It Have to Be Snakes?: Erik Broad-Shoulders and Erik the Rash are both afraid of the dark, which is why Olaf has them guard the ship while the rest of the Vikings raid a village.