Grady Tucker and his sister, Emily, are forced to move to Fever Swamp in Florida because of their father. Their father is a scientist attempting to study the effects of a Floridian swamp's environment on a group of deer, which he keeps in a pen outside of his family's new house. The siblings decide to explore the swamp, but they quickly become lost. Eventually, they come across an old shack. The two enter to see if there's anyone inside who can help them. However, they are scared away by the crazy looking hermit that lives there. The kids run home to tell their parents what happened, and their father tells them that the old hermit was purely harmless.

That night, while Grady is hanging out by his family's new house, he is befriended by a local boy named Will Blake. As Grady learns from Will, the only other local kid close to their age is a strange girl, but Will doesn't elaborate on this. Will also tells Grady about how the swamp was named. According to him, many years ago, the residents of the swamp all came down with a terrible fever. The fever caused madness or death in everyone who had it.

Eventually, the boys part ways, and Grady goes back inside. At night, as Grady tries to sleep, he hears howling coming from somewhere outdoors. The following morning, he decides to go outside. A dog jumps out at him, but it appears to be friendly. Grady's father allows him to keep the dog, and Grady names it Wolf because it looks like it might be part-wolf. The Tuckers decide that Wolf was probably the one causing the noises they heard last night. Later, Grady and Will explore the swamp, and Wolf follows. The kids find a dead heron, which they are puzzled by. Grady goes home. That night, Grady and his family are awoken by a crash from within the house. It turns out that Wolf was running about and bumping into things. The family doesn't really have any other choice aside from letting the dog outside, which they do.

The next day comes, and Grady goes outside. Wolf returns, but Grady is shocked to discover a badly mutilated rabbit by their home. His family is quick to blame Wolf, but Grady defends his dog, not believing that he was the culprit. Later in the day, Grady meets Cassie O'Rourke, the strange local girl that Will mentioned. Cassie tells Grady about the werewolf of Fever Swamp, explaining that the swamp hermit is a werewolf. Then, the children are approached by the hermit. Wolf growls at the man. The hermit's shirt is covered in blood, he's carrying a dead turkey, and he cries out, claiming to be the werewolf. The kids try to run away, but the man catches up to them. The hermit admits that he had been hunting and had wanted to play a joke on the kids. Before separating, the old man warns Grady about trusting Wolf.

That night, Grady hears something loud happening outside. As is discovered later, one of Dr. Tucker's deer was killed in its pen by an unknown creature. Dr. Tucker believes that Wolf is the problem, and thinks he should be dealt with. When Dr. Tucker reveals his plans of taking Wolf to the pound, Grady sets the dog free into the woods. Grady doesn't see Wolf again until that night, when he sees the dog roaming nearby the house, so he goes to investigate. Grady quietly sneaks outside. In the swamp, he discovers that Will is also sneaking about. Will says that he was investigating some howling that he heard. Grady tries to stick with Will, but he loses track of the boy. Suddenly, a werewolf jumps out at Grady. As the creature attacks, Grady recognizes the distorted, monstrous face, thick neck, and broad shoulders. He can tell that the werewolf is Will. The monster bites Grady, but Wolf appears and attacks Will, scaring him off. Grady falls unconscious.

When Grady regains consciousness, he's back home. As he learns, the swamp hermit carried him back to safety. Grady tells his father about what happened in the swamp, so Dr. Tucker decides to check up on Will. When he goes to the house where Will is supposed to live, he finds the house unoccupied.

A month passes, and Grady is staring up at a full moon. When Will bit him, Grady became a werewolf. Now whenever there is a full moon, Grady goes hunting in the swamp. The book ends with Grady running side by side with Wolf into the swamp.

The werewolf is a staple of supernatural fiction, whether it be film, television, or literature. You might think this snarling creature is a creation of the Medieval and Early Modern periods, a result of the superstitions surrounding magic and witchcraft.

In reality, the werewolf is far older than that. The earliest surviving example of man-to-wolf transformation is found in The Epic of Gilgamesh from around 2,100 BC. However, the werewolf as we now know it first appeared in ancient Greece and Rome, in ethnographic, poetic and philosophical texts.

These stories of the transformed beast are usually mythological, although some have a basis in local histories, religions and cults. In 425 BC, Greek historian Herodotus described the Neuri, a nomadic tribe of magical men who changed into wolf shapes for several days of the year. The Neuri were from Scythia, land that is now part of Russia. Using wolf skins for warmth is not outside the realm of possibility for inhabitants of such a harsh climate: this is likely the reason Herodotus described their practice as “transformation”.

The werewolf myth became integrated with the local history of Arcadia, a region of Greece. Here, Zeus was worshipped as Lycaean Zeus (“Wolf Zeus”). In 380 BC, Greek philosopher Plato told a story in the Republic about the “protector-turned-tyrant” of the shrine of Lycaean Zeus. In this short passage, the character Socrates remarks: “The story goes that he who tastes of the one bit of human entrails minced up with those of other victims is inevitably transformed into a wolf.”

Literary evidence suggests cult members mixed human flesh into their ritual sacrifice to Zeus. Both Pliny the Elder and Pausanias discuss the participation of a young athlete, Damarchus, in the Arcadian sacrifice of an adolescent boy: when Damarchus was compelled to taste the entrails of the young boy, he was transformed into a wolf for nine years. Recent archaeological evidence suggests that human sacrifice may have been practised at this site.