When a corpse with webbed feet and other aquatic adaptations washes ashore during a hurricane, the county medical examiner calls in marine biologist Will Myers for assistance. The deceased's mysterious sister, Andreia, claims the body and asks Will to help figure out how her brother died. Will and Andreia bond over shared tragedies and a yearning to heal a dying ocean as they seek to learn how her brother spent his final days.

Andreia brings Will to her undersea home, part of a hidden civilization inhabited by smugglers, hackers, treasure hunters, and traders—all members of a different species, driven to the edge of extinction by human diseases and climate change. As feelings between the two grow, the investigation into her brother's death leads to a sinister plot by a fanatical cabal. Together, Will and Andreia must find a way to save both humanity and the ocean without imperiling the existence of her species.