A cosmic mix-up results in a Martian baby being delivered to Earth, while an Earth baby is sent to Mars. Joseph Wilbur and his wife try to raise the green-skinned, ingenious Martian tyke as if he were an Earthling. But the kid builds his own spaceship and flies away, and Wilbur must find him and bring him back, or hell never be able to make an exchange with the Martian parents for his own boy.