On a damp night in 1722, Babylou Mac and her three siblings witness the murder of their mother at the hands of the preacher's son—so Babylou kills him. With plantation dogs on their heels, the siblings breach the treacherous confines of the Great Dismal Swamp. Deeper and deeper into Dismal they delve, toward a refuge where they can live freely within the swamp's natural—and supernatural—protection.

Three-hundred years later, college student Atlas comes home to North Carolina for the annual Bornday cookout and hog roast: a celebration of the fact that she and her three cousins were all born on the same day nineteen years ago, sharing a birthday with their Grannylou. But this Bornday, Grannylou's usual riddles and folktales about a marvelous paradise deep in the Great Dismal Swamp start to take on a tangible quality.

When Dismal calls, sucking Grannylou in, it's up to Atlas and her cousins to uncover the history that the black waters hold. Centuries of family tension are about to be dug up. Because Babylou and Grannylou are one and the same, and the power she helped cultivate hundreds of years ago—steeped in Black resistance, familial love, and the otherworldly mysteries of the Great Dismal Swamp—is bubbling back up. But so is a bitterness that runs deep as the swamp's waters. And some are ready to take what they feel they're owed.