Part I: The Marsh
In 1952, six-year-old Catherine Danielle Clark (nicknamed "Kya") watches her mother abandon her and her family due to violent abuse from her husband, Kya's father. While Kya waits in vain for her mother's return, she witnesses her older siblings, Missy, Murph, Mandy, and Jodie, all leave as well, due to their father's drinking and physical abuse. After her mother leaves, her father burns most of her mother's wardrobe and paintings.
Alone with her father—who temporarily stops drinking—Kya learns to fish. Her father gives her his knapsack to hold her collections of shells and feathers. The illiterate Kya paints these shells and feathers, as well as the marsh's creatures and shorelines, with watercolors her mother left behind.
One day Kya finds a letter from her mother in the mailbox and leaves it for her father to find, as she cannot read. When she returns to their home, she finds that he has burned the letter. He returns to drinking and takes long, frequent trips away to gamble. Eventually, he does not return at all, and Kya assumes he is dead, making him the last of the family to leave her alone in the marsh. Without money and family, she survives by gardening and trading fresh mussels and smoked fish for money and gas from Jumpin', a black man who owns a gasoline station at the boat dock. Jumpin' and his wife Mabel become lifelong friends to Kya, and Mabel collects donated clothing for her.
As Kya grows up, she faces prejudice from the townspeople of Barkley Cove, North Carolina, who nickname her "The Marsh Girl." She is laughed at by the schoolchildren on the only day she goes to school and is called "nasty" and "filthy" by a pastor's wife. However, she becomes friendly with Tate Walker, an old friend of Jodie's who sometimes fishes in the marsh. When Kya loses her bearings one day, Tate leads her home in his boat. Years later, he leaves her feathers from rare birds, then teaches her how to read and write. The two form a romantic relationship until Tate leaves for college at the University of North Carolina at Chapel Hill. He promises to return, yet later realizes Kya cannot live in his more civilized world because of how wild and independent she is, and leaves her without saying goodbye.
Part II: The Swamp
Delia Owens's novel is set in a North Carolina marsh like the one illustrated above.In 1965, Kya is 19. Chase Andrews, Barkley Cove's star quarterback and playboy, invites her to a picnic, during which he tries to have sex with her. He later apologizes, and the two form a romantic relationship. He shows her an abandoned fire tower, and she gives him a necklace of a shell he found during their picnic, strung on a rawhide string. Despite her suspicions, she believes Chase's promises of marriage and consummates their relationship in a cheap motel room in Asheville, North Carolina. After seeing in a newspaper that Chase has become engaged, Kya realizes his promises of marriage were a ruse. She then ends their relationship.
Tate, having graduated from college, visits Kya and attempts to apologize for having left her and confesses his love for her. Still hurt from his betrayal, she rejects him. Despite this, she allows him inside her shack, and he is impressed by her expanded collection of seashells. He urges her to publish a reference book on seashells, and she does so as well as on seabirds. With the extra money, she renovates her home. The same year, Jodie, now in the Army, also returns to Kya's life, expressing regret he left her alone and breaking the news their mother had suffered from mental illness and died of leukemia two years previously. Kya forgives her mother for leaving but still cannot understand why she never returned. After advising Kya to give Tate a second chance, Jodie sets off for Georgia, leaving Kya a note with his phone number and address.
Some time later, while relaxing in a cove, Kya is confronted by Chase. After an argument ensues, Chase attacks Kya, beating her and attempting to rape her. She fends him off and loudly threatens to kill him if he doesn't leave her alone. The encounter is witnessed by two fishermen nearby. Back at her shack, Kya fears that reporting the assault would be futile as the town would blame her for "being loose". The next week, she witnesses Chase boating up to her shack and hides until he leaves. Remembering her father's abuse, Kya fears retaliation from Chase, knowing "these men had to have the last punch".
Kya is offered a chance to meet her publisher in Greenville, North Carolina, and takes the bus there to meet him. After she returns home the next day, some young boys find Chase dead beneath the fire tower. The sheriff, Ed Jackson, believes it to be a murder on the basis of there being no tracks or fingerprints, including Chase's, around the tower. Ed speaks with sources and receives conflicting statements. He learns the shell necklace Kya gave to Chase was missing when his body was found, even though he wore it the night he died. Kya was seen leaving Barkley Cove before the murder, then returning the day after Chase died. There also were red wool fibers on Chase's jacket that belonged to a hat Tate had given to Kya. Convinced she is the culprit, Ed arrests Kya near Jumpin's wharf, charges her with first-degree murder, and jails her without bail for two months.
At Kya's trial in 1970, only contradictory and circumstantial evidence is provided. Kya's lawyer, Tom Milton, debunks the prosecutor's arguments as there was no evidence that Kya was at the fire tower on the night of Chase's death. The jury finds her not guilty. She returns home and reconciles with Tate. They live together in her shack until she dies peacefully in her boat at the age of 64. Later, while searching for Kya's will and other documents, Tate finds a hidden box with some of her old possessions and poems she had written. He finds a poem that alludes to Chase's murder, and, underneath the poems, the shell necklace Chase wore. Tate burns the poems and the rawhide string before dropping the shell onto the beach. Kya is buried on her property near the shack.
