When Adra Dantes’ half-brother Cameron attacks their father and robs him of a priceless treasure map, he leaves him a shell of the pirate captain he once was. Now Adra’s only aim in life is to kill Cameron, retrieve her father's map, and claim the treasure herself.

But her plans are thwarted. A sudden surplus of magic in the world is causing ancient sea monsters to awaken. Worse yet, Adra discovers that the ship she's been chasing for almost a year now is captained by a girl who’s been impersonating Cameron, while Cameron himself is missing.

The two pirate captains will have to work together if they are to find Cameron, but before they can do so, they must vanquish the beast―known as The Devourer―that is sinking ships and causing so much fear. Adra will have to descend leagues beneath the sea to the creature’s lair to strike a deal with her, but she’ll discover she isn’t the only one looking for her brother―and what he stole.