Aliens will take possession of Earth unless John Trent can defeat their champion in a game of survival. Though out-gunned, Trent uses his wits to protect the world.
Thad Merkel invents a machine to mass-produce copies of things like cars and planes, but the machine seems to have a childlike mind of its own. This playfulness helps the inventor stop a group of thieves, but the idea of a self-aware machine is too dangerous, and Merkel destroys its master circuit.
Working on a way to create a barrier against nuclear fallout, two scientists are visited by creatures accusing humans of having stolen their world. Eventually the creatures realize they are not on their planet and the scientists reverse their experiment, sending the creatures back to their universe.