Grady and Emily move to Florida after their parents accept a new research assignment for the university. Their dad brought in some weird deer from South America and want to see how they fare in the US. They plan to put tracking devices on them and follow their movements. Grady doesn't really mind the move, but Emily hates that she has to leave all her friends behind and miss out on her last year in high school.
In the hopes of having some fun, they take a walk in the swamp behind their house. They stumble across a random cabin in the woods and some man who mumbles to himself and seems a little crazy. When he chases them away from the cabin, they run home and tell their parents what happened. In a fit of Wakefield-like parenting, their dad tells them not to worry. He heard in town about a crazy hermit living in the swamp but it's not a big deal.
Not long after moving there, Grady meets this kid Will. They take some walks and explore the swamps, and he eventually introduces him to this girl Cassie. Cassie is obsessed with the idea of werewolves and thinks there is at least one living in the swamp. Will also tells him that Fever Swamp got its name after a weird virus swept through the area and killed a bunch of people. Not surprisingly, Grady later comes down with a fever of his own. It tends to come and go, but his parents just ignore it.
Then, he and Emily both wake in the middle of the night to the sounds of something outside howling and making loud noises. Their parents wake and claim it was just a side effect of his fever, even though Emily heard it too. The next day, they find a random dog outside their house. Everyone decides that the dog was responsible for the noises they heard. Both kids beg their parents to keep the dog, and since it looks like a wolf, they name him Wolf.
The first night Wolf stays in the house, he goes crazy. He runs around the house, breaks a lamp, and keeps howling, so they put him outside. Several random animals then turn up dead. They find a heron, a rabbit, and then one of the deer. Grady's dad decides that Wolf is too much of a danger and that he should go to the pound. Grady disagrees.
He, Will, and Cassie go off to check out the woods. She keeps talking about werewolves and they try to ignore her. They come across the hermit, who goes cray-cray and starts screaming that he is the werewolf. The kids take off, and Grady gets separated from the group. After being bit by a snake in the woods, Wolf licked his hand until he felt better while his parents just ignored him. When his dad comes for his dog, he pushes him away, tells him to run, and Wolf takes off.
When Grady hears howling again in the middle of the night, he decides to investigate on his own. He runs into Will in the swamp, and they decide that it must be the hermit. Before they can get there though, Will transforms into a werewolf and bites Grady. Wolf attacks, the two fight, and Grady passes out.
He wakes up at home and finds out that the hermit brought him home after finding him in the woods. Grady tells them all about Will and where he lives. His dad assumes it was just the fever talking again but agrees to check things out. He comes back with the news that no one lives in the house or has lived there in a long time. Since the hermit claims he saw Wolf chasing off another animal, they decide to let him keep his dog. And then, in a move that shouldn't surprise anyone, Grady reveals that he's now a werewolf and prowls the woods with his dog by his side.
*My copy is one of the newer ones from the mid-2000s, and it has a bunch of extra stuff in the back, like an interview with Stine himself. It's kind of weird to read about him surfing the web and playing games online.
*The description on the back of the book talks about how Grady's dog howls at the moon, disappears at midnight, and all this other crap that doesn't really happen in the book.
*Is this the first time Stine used a teenager in a Goosebumps book? I can't remember anyone other than Emily having a bigger role.