The body of Chase Andrews is found by two boys next to an abandoned fire tower in the swamp on October 30, 1969.

In 1952, six-year-old Kya watches her mother leave their shack in the marsh. Before long, her siblings, including her beloved 12-year-old brother Jodie, depart too, leaving Kya alone with Pa, a drunken, disabled World War II vet prone to violent rages. Pa gives Kya a dollar a week for food. In town, older boys, including Chase Andrews, pass her on their bikes. Kya spends most of her time in the marsh with the gulls.

Mrs. Culpepper, a truant officer, takes Kya to school, where she is humiliated and never returns. Over the years, townspeople call her “dirty,” “the Marsh Girl,” “Missing Link,” and “marsh trash.”

Kya takes Pa’s fishing boat exploring. When she gets lost, one of Jodie’s friends, Tate Walker, guides her back to her channel. Tate lives alone in town with his dad, Scupper, a fisherman. 

For a while, Kya and Pa fish together and enjoy each other’s company. Pa introduces Kya to Jumpin’, an older Black man who sells gas and supplies. Kya’s mother sends a letter, but Pa burns it and returns to drinking. Pa finally disappears when Kya is ten years old. Kya supports herself selling mussels to Jumpin’, whose wife Mabel collects clothes for Kya from Colored Town. Kya continues watching Tate as well as Chase and his friends.

When she is 14 years old, Kya finds a series of special feathers left in a tree stump in the marsh. One day, she leaves a special feather in return. After that, Tate appears at the stump and offers to teach her to read. Tate and Kya begin to meet at a secret cabin in the marsh where he teaches her to read. As Kya matures, Tate brings her a biology textbook and Mabel gives her a bra. One day Tate finds Kya sick on Point Beach. He explains to her that she is having her period. Kya and Tate continue their lessons and she invites him inside her shack and shows him her collection of natural objects. One day, Tate kisses Kya and they begin a romantic relationship. Their desire grows but they avoid having sex. Tate leaves early for college and tells Kya he will visit on the 4th of July. However, he doesn’t return. Kya, devastated, returns to studying nature.

When Kya is 19 years old, Chase catches her eye on Point Beach. He is a popular, good looking, confident, former high school quarterback. Chase approaches Kya on Jumpin’s wharf and invites her on a picnic. 

At the picnic, Chase gives Kya a rare scallop shell. He is sexually aggressive and she runs away. When Kya and Chase meet ten days later, he apologizes and takes her to see the abandoned fire tower. Kya gives Chase a necklace made from the scallop shell. Chase says he won’t approach her sexually unless she wants him to.

Kya gets closer to Chase. Tate, who is home from college, wants to apologize to Kya, but sees her kissing Chase and turns away. Chase starts talking about marriage and invites Kya on an overnight trip. She joins him and they have unsatisfying sex in a cheap motel room. Still, their relationship continues. However, Chase won’t introduce Kya to his parents or friends and disappears for a week over Christmas. 

Tate comes to the lagoon to warn Kya that Chase is seeing other women, but she throws rocks at him. Still, she invites him into the shack and he offers to find a publisher for a book of her nature paintings. 

Kya sees the engagement announcement in the newspaper for Chase and Pearl, a girl she recognizes from Point Beach. She recites poems by Amanda Hamilton, who publishes in the local newspaper, to soothe herself. She determines to live her life alone.

A year later, Kya receives a copy of her first book. She uses the advance money to improve and modernize the shack. She gives Tate a copy of her book. 

Kya’s brother Jodie visits the shack. He tells her that Ma has died and promises to keep in touch. He encourages her to reconcile with Tate.

In August of 1969, Chase sneaks up on Kya on a remote beach, attacks and tries to rape her. She fights him off and sees two fishermen watching from their boat as she leaves. Tate and Jumpin’ see her bruises, but she begs them not to tell anyone. Chase continues to come to her shack, but she hides from him. Kya gets a letter from her publisher, Robert Foster, to meet him in Greenville. She finds out from Tate how to buy a bus ticket and goes to meet him on October 28, returning on October 30, the day Chase’s body is found.

Sheriff Ed Jackson and his deputy Joe Perdue investigate Chase’s death. No footprints or fingerprints are found at the scene. They think Chase was pushed through an open gate and the murderer covered up the scene. Almost immediately, everyone in town suspects “the Marsh Girl.” 

Over time, Joe and Ed’s main clue consists of red wool fibers on Chase’s jacket. Also, Chase’s mother Patti Love tells them Chase was wearing the shell necklace when he left her house that night, but it was not found at the scene. A shrimper, Hal Miller, tells them he saw Kya’s boat headed toward the fire tower the night Chase died. However, Tate and Jumpin’ and townspeople who saw her get on and off the bus to Greenville give Kya an alibi. A fisherman, Rodney Horn, who heard Kya yelling and saw her leaving after Chase attacked her, heard her say she would kill Chase if he bothered her again. Joe and Ed find a red wool hat in Kya’s shack that Tate gave her and that matches the fibers on Chase’s coat. With this information, they arrest Kya for murder.

Kya spends two months in jail awaiting trial. A lawyer, Tom Milton, volunteers to defend her. 
A kind guard at the prison, Jacob, lets the cat, Sunday Justice, into her cell for company. Tate, Jumpin’, and Mabel sit behind Kya in court for support. They are later joined by Jodie, Robert Foster, and Scupper. 

Prosecutor Eric Chastain calls his witnesses and Tom Milton counters their testimony. The jury learns that it would have been possible for Kya to take buses back and forth from Greenville on the night of the murder, but the timing is very tight. Tom calls defense witnesses, including Kya’s publisher Robert Foster. After closing statements, everyone awaits the verdict. Kya is found not guilty. Jodie takes her home, where she is delighted to see the marsh again. 

Kya sees Tate in his boat and is going to approach him, but the sheriff and two deputies arrive and take Tate with them. She learns that Scupper has had a stroke and died.

The day after Scupper’s funeral, Tate finds a feather in his boat from Kya. At the shack, they express their love for each other and Tate moves in with her. Over the years, Jumpin’ dies, and Jodie and his wife and children visit the shack. Tate works at the nearby lab and Kya writes seven more books. 

One day Tate finds Kya in her boat, dead of a heart attack at age 64. On her tombstone, he inscribes “The Marsh Girl.” Many townspeople come to Kya’s funeral on her land. That night, Tate finds two items beneath the floorboards. One is a box of poems by Amanda Hamilton, a pseudonym Kya used, including a poem describing Chase Andrews’ death. Tate also finds the shell necklace Kya gave Chase. Tate burns the poems and rawhide cord and crushes the shell on the beach for the tide to take away.