When Harry Altman and his brother, Alex get off the bus to Camp Spirit Moon, they are horrified to discover there's not a single person in sight. However, before the two can panic, a counselor comes out of the woods and reveals it was all a practical joke. The two are then introduced to Uncle Marv, the camp director, a large, intimidating-looking man. Uncle Marv welcomes the two, and begins to explain some of the camp traditions, including their official greeting. He vaguely tells them about a welcoming campfire that all new campers have to attend, which he deems the true test that will prove they belong at camp.
When the campfire ceremony comes, Uncle Marv sits down and tells the campers two ghost stories. One is about the story of the ghost camp. Uncle Marv tells them that long ago, a mysterious fog crept into a campground, turning everyone there into ghosts. While Harry and Alex aren't frightened by his stories, the rest of the campers are.
When the campfire ends, strange things start to happen. Harry finds puddles of cold, blue slime in his cabin. Then, the campers start to play terrifying, and seemingly implausible pranks on them. One boy sticks a fork in his neck, and another's head seems to fall off during a soccer game. Harry begins to question if the ghost stories could be true. Eventually, it is revealed they are. Every camper is a ghost, and needs a body to possess in order to escape the camp for good. In the book's final chapters, Lucy is about to possess Harry, but Uncle Marv and the other campers pull her away and try to possess him instead. All of the ghosts begin fighting, causing them to blend together into a smokey mist. The mist dissipates into the night sky, taking Uncle Marv with it.
While Uncle Marv appears to be a kind and jolly man at first, he shows his true nature later on. Like the rest of the campers, he wanted to possess either Harry or Alex in order to finally escape the camp.
The book compares Uncle Marv to a grizzly bear. He's a large man, with long, greasy black hair, bushy eyebrows, and tiny blue eyes.