E.J. Mackernutt, Jr., an 11-year-old boy who has always been fascinated by space and astronauts, sneaks into Cape Kennedy and becomes a stowaway on a spaceship scheduled for a lunar landing. After he is discovered, NASA at first cancels the landing, but E.J. and the astronauts convince them to go forward with it. Problems arise when one of the astronauts is incapacitated by illness and the other two are stranded on the moon's surface, but E.J. is able to command the space capsule and save the day.