Earth has had interstellar spaceflight for centuries and has made contact with numerous extraterrestrial intelligent species. John Thomas Stuart XI, the teenage protagonist, lives in a small Rocky Mountain town, Westville, caring for Lummox, an extraterrestrial beast his great-grandfather had brought home. Lummox has learned how to speak, and has gradually grown from the size of a collie pup to a ridable behemoth—especially after consuming a used car. The childlike Lummox is perceived to be a neighborhood nuisance and, upon leaving the Stuart property one day, causes substantial property damage across the city. John's widowed mother wants him to get rid of it, and brings an action in the local court to have it destroyed.Desperate to save his pet, John Thomas considers selling Lummox to a zoo. He rapidly changes his mind and runs away from home, riding into the nearby wilderness on Lummox's back. His girlfriend Betty Sorenson joins him and suggests bringing the beast back into town and hiding it in a neighbor's greenhouse. However, it is not easy to conceal such a large creature. Eventually, the court orders Lummox destroyed. City officials try several methods to kill Lummox but fail, as his alien physiology appears to be virtually invulnerable to ordinary weapons or poisons, and Lummox does not even realize they are attempting to execute him.