In 1973 two boys find the body of the scion of a local important family (Jodie) dead at the foot of a fire-tower in the N.Carolina marshlands. The sheriff investigates and arrests the "marsh girl" who lives alone in the swamplands for murder.

Her back story comes out as she tells it to her lawyer

Kya (Daisy Edgar-Jones) aged 7 is the youngest of 4 children in a family living in a shack in the swamps. Dad is an abusive drunk who drives first Ma, then each of her older siblings to leave. She learns to live with her father by avoiding him a lot. Eventually Dad leaves, abandoning her and she continues to live alone in the shack pretending to the locls she encounters that her dad is still around.

She grows up as a nature child and meets a sympathetic local boy, Tate (Taylor John Smith), who understands her and teaches her to read from her mothers library of natural history books. Over the years they fall in love and she develops as an artist and naturalist. 

Tate is slated to go to NCU in Chapel Hill to study biology and become a research scientist. He leaves promising to come back for 4th July. He doesn't.

Kya is devastated. Eventually she is picked up by Chase (Harris Dickinson), son of a wealthy local family with all the advantages in life and wants the Marsh Girl as a trophy.  Inevitably there is a falling out between them when she discovers he is already engaged and has been leading her on (although she hasn't given in apart from an unsatisfactory night in a motel). He continues to pursue her, meanwhile she, prompted by something Tate said before he left, has send her paintings and words to a publisher and had them accepted for a book. The book is published and Chase renews his assault and goes wild when she rejects him. He is then found dead at the foot of the tower.

The courtroom drama has been intercut with this and the film now becomes a fairly straightforward courtroom drama as her lawyer manages to get her acquitted.

Tate and Kya get back together and have a long and happy life living together and pursuing their mutual interst in the natural world as a scientist (Tate) and Artist (Kya). In her 70s Kya dies and Tate discovers a secret that changes everything. A secret he promptly consigns to the waters.

The details of the book which I read a couple of years before seeing the film are here