Bored after their previous adventure, Tom Sawyer concocts a scheme to bamboozle the villagers of St. Petersburg into believing that Bigfoot-type monsters are migrating up the Mississippi River. He and his friends, Huck, Becky, Jim, and Zane Rasmussen, a teen time-traveler from the 21st century, fake footprints and blood evidence to claim that they have killed one of the monsters on an island and saved the village. They are heroes once again. But their plot is upstaged when a man is found mauled by an unknown creature on an island and huge footprints are discovered. The co-conspirators are chilled to realize they might be dealing with a real monster. The youngsters, the law and the villains gradually come together in a crashing climax.