Eighteen-year-old Cassandra Leung struggles with her morality and her romantic relationship with fellow pirate Swift Kent as she and the Minnow pirate crew work to take down wild sea monsters, dubbed Hellbeasts, who are attacking ships and destroying the ocean ecosystem.

Cassandra Leung--a seventeen-year-old trainer of Reckoners, sea beasts bred to defend ships--is kidnapped by the pirate queen Santa Elena and ordered to train a Reckoner pup to defend Santa Elena's ship.