Since the hurricane, the town of Mercy, Louisiana has been overtaken by a strange red algae bloom. Noon and her mother have carved out a life in the wreckage, trawling for the mutated wildlife that lurks in the water and trading it to the corrupt harbormaster. When she's focused on survival, Noon doesn't have to cope with what happened to her at the Cove or the monster itching at her skin.

Mercy has never been a safe place, but it's getting worse. People are disappearing, and the only clues as to why are whispers of underwater shadows and warnings to never answer the knocks at night. When the harbormaster demands she capture the creature that's been drowning residents, Noon finds a reluctant ally in his daughter Covey. And as the next storm approaches, the two set off to find what's haunting Mercy. After all, Noon is no stranger to monsters . . .