Gregg Haljan was aware that there was a certain danger in having the giant spaceship Planetara stop off at the moon to pick up Grantline's special cargo of moon ore. For that rare metal--invaluable in keeping Earth's technology running--was the target of many greedy eyes. But nevertheless he hadn't figured on the special twist the clever Martian brigands would use. So when he found both the ship and himself suddenly in their hands, he knew that there was only one way in which he could hope to save that cargo and his own secret--that would be by turning space-pirate himself and paying the Brigands of the Moon back in their own interplanetary coin.

There were nine major planets in the Solar System and it was within their boundaries that man first set up interplanetary commerce and began trading with the ancient Martian civilization. And then they discovered a tenth planet--a maverick! This tenth world, if it had an orbit, had a strange one, for it was heading inwards from interstellar space, heading close to the Earth-Mars spaceways, upsetting astronautic calculations and raising turmoil on the two inhabited worlds. But even so none suspected then just how much trouble this new world would make. For it was WANDL THE INVADER and it was no barren planetoid. It was a manned world, manned by minds and monsters and traveling into our system with a purpose beyond that of astronomical accident!

Lee Anthony finds himself and two of his friends kidnapped and taken on a strange voyage. (Summary by Richard Kilmer )