Cassandra Leung (Cas), the protagonist of the story, has always wanted to be a full Reckoner trainer, but although she began learning to train the monsters as a child, she has never had full control of them as they defend ships against the pirates that roam and pillage the seas of the NeoPacific. At 17, Cas finally gets her chance. She is named the trainer of Durga, a large turtle-resembling Reckoner who's in charge of a cruise ship, an easy ship to protect. But, instead of smooth sailing, something happens to Durga, and Cassandra is captured by a pirate ship helmed by the evil and complicated, Santa Elena, and her crew of equally complex mutineers. Somehow, Santa Elena has captured a Reckoner of her own and needs Cas to raise it in order to level the playing field on the high seas.
Cas is held as a prisoner-like entity on the ship (she doesn't get special privileges, but she isn't locked up) and comes to meet the various members of the crew that includes a group of five teenagers (Chuck, Code, Lemon, Varma, and Swift) who are in the running to become Santa Elena's successor. Each of the trainees has a special skill that has caught the captain's eye, but Swift is at the head of the ranks, which is why Elena gives her the most difficult job of all - to ensure the Reckoner is raised properly and that Cas doesn't try to flee. If Swift fails at either task, her life is also at risk.
The bonding of Cas and Swift's lives creates a relationship that neither girl anticipated, and it forces Cas to rethink all she knows about pirates and their lives on the sea. There is much more to the world than what she learned as a citizen of the Southern Republic of California, and she doesn't realize just how privileged her life has been until she is forced to look at the lives of those who live differently.
What I like most about the novel is Cas' consistent musings about the gray areas of morality. She has learned how to train Reckoners to kill pirates and destroy their ships, which she sees as ethically just because of the stereotypes surrounding pirate life and the laws that society has put in place that specifically target pirates. Once on the ship, however, she is presented with the argument that she is just as ruthless as the pirates, possibly more ruthless, because although the pirates pillage and plunder for a living, at least they only kill in self-defense - the people on the ship who do not resist are allowed to live. Additionally, Cas struggles with where she belongs in the world. She thinks the 'right' thing to do would be to flee the pirate ship and go back to her family, but in a society where they give trainers a cyanide-type pill before setting out to sea just in case they are captured by pirates (literally, her father gives her the pill and tells her to take it because she holds special trainer secrets), it's hard to know where home is. Lastly, she struggles with her relationship with Swift. Swift is her captor, but she is also the girl who saves her life.
- Women are the prominent characters in the novel - the captain of the ship is Santa Elena, who took control of her ship with a baby on her back; Cassandra's mother is the engineer of the monsters, while her father trains them; Swift seems to take the lead position in the contest to become the next captain of the ship
- Diverse Characters:
- Cassandra (Cas) = Asian, lesbian girl
- Swift = white, lesbian girl
- Varma = Hindu boy;
- Chuck = Pacific Islander girl;
- Lemon = Aleutian?
- Captain Santa Elena = ambiguous in race/ethnicity, but she's described as having brown skin
- There's a father who is raising four children and taking care of his mother. He can't work because he's taking care of everyone, so he relies on outside funds to help him to take care of the family, which turns the trope of the single mother on its head.