One day in 1952, six-year-old Kya Clark’s mother walks down the lane leading from their rundown shack in the North Carolina marshlands. Kya senses that Ma won’t return, but her brother, Jodie, assures Kya that she will, claiming that mothers only abandon their children in order to survive—a reason he doesn’t think pertains to Ma. However, Ma never returns, leaving Jodie, Kya, and their three older siblings with their violent and alcoholic father. As time passes, Kya and Jodie’s siblings leave, too, and then Jodie also abandons the shack. Suddenly, then, Kya is on her own with Pa, wondering how she’s going to care for herself. With this in mind, she teaches herself how to cook, slowly learning how to prepare grits and taking it upon herself to do the shopping in the nearby town of Barkley Cove, though she dislikes going into public because the townspeople mistreat her and call her “swamp trash.” Still, Kya learns how to sustain herself, and she even manages to mollify Pa’s anger by cooking for him. Because of this, he takes her fishing in the marsh and introduces her to Jumpin’, who owns a nearby gas dock and bait shop. Jumpin’ is one of the only people Kya interacts with other than Pa, though she also has fleeting encounters in the marsh with a boy named Tate.
The narrative jumps back and forth between Kya’s upbringing and a murder case involving a young man named Chase Andrews. This case takes place in 1969, when two local boys find Chase’s dead body beneath the fire tower in the marsh. When Sheriff Ed Jackson comes to investigate, he notices that there aren’t any footprints in the mud around Chase’s body, suggesting that Chase was murdered. As the novel progresses, the storyline switches between Kya’s coming of age process and Ed’s investigation, in which he becomes certain that Chase was murdered and begins to suspect that Kya has something to do with what happened, since she and Chase had a secret relationship in the marsh until just before Chase married a local girl named Pearl.
As a child, Kya only attends school for one day, refusing to go back after being bullied by the other children who mock her and call her a “swamp rat.” However, despite a lack of formal education, she becomes quite self-sufficient. Kya even starts enjoying spending time with Pa—this changes, though, when Pa receives a letter from Ma. Kya doesn’t know what the letter says because she can’t read, but whatever it says enrages Pa, who burns it before going out to get drunk. This marks his return to his abusive, alcoholic ways, and it isn’t long before Pa starts disappearing for long stretches of time. Then, when Kya is 10, Pa leaves and never comes home, leaving Kya completely on her own. To make money, Kya starts collecting mussels and selling them to Jumpin’, but because there are other fishermen who sometimes sell him mussels before her, she tries to think of different ways to make money. After catching and smoking a large quantity of fish, Kya agrees to give them to Jumpin’ on consignment. However, Jumpin’ takes the entire bucket of fish home, knowing they won’t sell. At home, his wife, Mabel, decides to make stew with the fish, but she instructs Jumpin’ to tell Kya that they found somebody who wants to trade clothes and other supplies for her fish. This isn’t actually true, but Mabel worries that Kya won’t accept their help if they don’t tell her this. Mabel then gathers a number of supplies and gives them to Kya, who is eternally grateful.
Around this time, Tate—who has taken an interest in Kya—starts leaving rare bird feathers for her on a stump in the woods. He admires Kya’s appreciation of nature, so this is a way to connect with her. He soon leaves other presents, too, including an extra spark plug for her boat’s engine and a short note. However, Kya can’t read his note, and she tells Tate this when he suddenly appears at the stump, catching her off-guard before she can run away. Kindly, he suggests that he could teach her to read, and she agrees. From that point on, they spend as much time together as possible, as Tate comes to the marsh and brings as many books as he can, relishing his time with Kya. Similarly, Kya comes to look forward to her time with Tate, and she also loves being able to read, especially since one of the books Tate gives her is an almanac about the kind of wildlife Kya sees in the marsh. Kya particularly loves poetry and connects with the poems of a local poet, Amanda Hamilton, that are published in the town paper.
Kya and Tate develop a romantic bond. Because Tate is 19 and Kya is still only 15, though, Tate refuses to have sex with her—a fact that frustrates her because she thinks he’s treating her as if she’s younger than she actually is. Still, he remains steadfast, though they do become physically romantic in other ways. Unfortunately for Kya, though, Tate informs her one day that he’ll soon be leaving, since he’s about to go to college and has just accepted a job for the summer in a biology research lab at school. Having said this, he promises to return on July 4th for a visit, so Kya eagerly awaits his arrival. When the day comes, though, Tate is nowhere to be seen, nor does he come the next day or the day after that. While waiting for him one evening, Kya watches a female firefly flash her light in a pattern that attracts a male of a different species. As soon as the male gets close, the female firefly eats him, and though Kya recognizes the mercilessness of this, she understands that this is simply how nature works—there is no morality when it comes to survival in the wilderness, she thinks.
On her own once again, Kya vows that she’ll never get close to anyone again. And yet, she can’t deny how lonely she feels, which is why she finds herself drawn to Chase when she spots him and his friends on the beach one day. As she watches him, he turns around and makes eye contact with her, and she can tell that he’s interested in getting to know her. Several days later, Kya sees Chase at Jumpin’s, and he invites her to go on a picnic in his boat that weekend. She agrees, and so they take his boat to a remote beach, where they go for a walk. At one point, Chase finds a shell that Kya identifies as somewhat rare, so he gives it to her before sitting down and playing harmonica. Then, after lunch, they kiss, and suddenly Chase throws his entire body onto Kya and tries to take off her clothes. Frantically, she tells him to stop and shifts out from underneath him, deciding to run home even though it’s quite far. But even though Kya is angry and feels violated, she later accepts Chase’s apology. To make it up to her, he takes her to the fire tower in the marsh and shows her the magnificent view, promising that he’ll never do anything to make her uncomfortable again. In turn, Kya accepts Chase’s apology and gives him a necklace she made using the shell he found on the beach. In the coming months and years, they continue to spend time together, eventually becoming romantic. And though Kya puts off having sex with Chase for as long as possible, she goes through with it after they’ve been seeing each other for several years. Overall, she finds it uncomfortable and unenjoyable, but Chase doesn’t seem to mind—he’s too satisfied to notice her discontent.
It is now 1966, and Tate has returned to the marsh, having finished undergraduate school and embarked upon a Ph.D. in protozoology. He wants to apologize to Kya for abandoning her, but she screams at him when he pays her a visit. Still, he tells her that he made the biggest mistake of his life by leaving her. Tate also tells Kya that Chase has been seeing other women in town, but she tells him that this is none of his business. Not knowing what else to say, Tate asks if he can come to Kya’s shack to see her collection of shells and feathers, and she hesitantly says yes. Once inside, he admires how the collection has grown since he last saw it and asks if he can take a sample to show several publishers, suggesting that Kya could publish a book containing her illustrations of the specimen she has amassed. She allows Tate to do this, and he takes his leave.
Kya doesn’t confront Chase about what Tate told her, but she does try to get him to integrate her into his life outside the marsh. Every time she brings this up, though, he puts off introducing her to his parents, all the while promising to someday marry her. One day, Kya goes into town and sees Chase with his arm around a young woman. She then buys a newspaper, and when she opens it at home, she sees that Chase and this woman—Pearl—have just announced their engagement. Kya promptly stops seeing Chase, and though she gets closer and closer to forgiving Tate, she doesn’t rekindle their romance.
A year after Kya breaks things off with Chase, he comes up behind her in a place called Cypress Cove. Forcing himself upon her, he tries to rape her, but she escapes by kicking him in the groin and then in the kidneys. As she runs back to her boat, she screams that she’ll kill him if he tries to harm her again—something a nearby fisherman, Rodney Horn, hears her say. This factors into Kya’s trial later that year, when she stands accused of having murdered Chase. Thankfully, Kya’ lawyer, Tom Milton, is quite good at refuting the state prosecutor’s allegations, primarily doing so by pointing out that the only indication that Chase was murdered is actually a lack of evidence. After all, there are no footprints, nor are there fingerprints on the fire tower, suggesting that somebody intentionally wiped them away. Throughout the trial, Eric—the state prosecutor—tries to frame Kya as the mysterious and sinister “Marsh Girl,” using various witnesses to present a narrative of her as emotionally unstable and dangerous. However, Tom manages to destabilize this argument by reminding the jury that Kya isn’t the fearsome creature the townspeople have made her out to be. Instead, he argues, she’s just a young woman who was abandoned in the marsh as a child and has had to fend for herself in order to survive. This argument ultimately resonates with the jury, which finds Kya not guilty of murdering Chase.
In the aftermath of Kya’s trial, she returns to her shack, having been held for several months in the courthouse’s jail. Throughout her trial, Tate sat behind her and gave her emotional support, as did Jodie, who returned to Kya shortly before Chase’s murder and told her he recently learned that Ma—who was apparently mentally unstable when she initially abandoned her family—died in New Orleans two years earlier. Jodie encourages Kya to forgive Tate, seeing quite clearly that she loves him. But until now, Kya has been too wary of letting him back into her life. Finally, though, she decides to reestablish their romantic connection. Once more, Tate apologizes for leaving her, and they move into her shack and live a happy life, as Tate works as a research biologist and Kya publishes seven books about the North Carolina wildlife. Then, while boating in the marsh one day, Kya slumps over and dies at the age of 64. After Kya’s funeral—which everyone in town attends—Tate looks for her will. Under their floorboards, he finds Chase’s shell necklace and a poem that Kya secretly wrote under the pseudonym Amanda Hamilton. It’s called “The Firefly,” and it reveals once and for all that she lured Chase to the fire tower and tricked him into falling through an open hatch.
