An adventurer calling himself the "Ace of Space"- stows aboard the Space Cadets' ship (the Ventura on this occasion) with the aid of the impulsive Roger. But "Ace" thereupon commandeers the ship and forces the Cadets to fly to Venus and help him search for the legendary Lost Colony. They do find it, amidst the Venusian jmgles, but they also encounter the very same menace that wiped out the original Colony, established by the first colonists from earth 200 years ago!