When Nolan Grey receives news that his father, a once-prominent biologist, has drowned off Leap's Island, he calls on Elsa, his estranged older half-sister, to help. This, despite the fact that it was he and Elsa who broke the family in the first place. Elsa and Nolan travel to their father's field station off the Gulf Coast, where a group called the Reversalists obsessively study the undowny bufflehead, a rare duck whose loss of waterproof feathers proves, they say, that evolution is running in reverse.

On an island that is always looking backward, it's impossible for the siblings to ignore their past, and years of family secrecy threaten to ruin them all over again. Yet, despite themselves, the Greys urgently trek the island to find the so-called Paradise Duck, their father's final obsession, all the while grappling with questions of nature and nurture, intimacy and betrayal, progress and forgiveness.