The Robinsons get a rocket from Earth with a needed gyro to find their way back home. But an explosion sends the rocket down to a prehistoric planet. The Robinson family (along with their pet dog Clancy) take the two spacemobiles down to recover it. Some cavemen take them prisoner, who have also made the rocket their idol. The Robinsons help them defeat a cave monster by giving them bows and arrows. They then are able to get the gyro package from the rocket and return to their space station.