He smiled at that and waggled a finger. "The plague is their protection, my son. Conquer that, and you will come upon the greatest treasure known to mankind. Listen...."
Well, I heard him out, patiently at first, then gradually with more and more interest. It was a madman's story in every detail, and yet there was something about it that got me. I knew how the seven cities of the High Ganymedian Plateau were first raided by Conway and his Earth Brigade after enjoying several thousand years' culture on this, the third satellite of Jupiter. How the captured emperor of the seven cities swore a curse of vengeance for the mishandling of his people and in some unknown way introduced the strange and terrible plague which was to turn the seven metropoli into pest-holes avoided and shunned by Earth and Jovian colonists alike.
Then Hol-Dai said something which made me prick up my ears. "Why," he said, "do you think the emperor introduced that plague? For vengeance alone? A ruler's vengeance does not go as far as dooming his people forever. No, my son, for another reason."
I said nothing, waiting for him to continue.
"For three thousand years the seven cities had been living off the plunder of conquered Io and Callisto, the first and second satellites. And never has it occurred to these fools what has become of that plunder."
"They probably will, Hol-Dai," I said. "Some day a fleet of space freighters will carry it all off."
The white-haired old man shook his head. "Not a fleet, my son. A man in the palm of his hand."
I sat down then, and I asked questions, and after a time I had the story in its entirety. Both Io and Callisto had been conquered by the people of Ganymede and had been forced to pay a huge indemnity. Part of that indemnity came in the form of a stone, called by the Ganymedians, the Jupiter Stone. That stone, protected by an envelope of white pinardium, contained a compressed particle of the light-active rock which formed Jupiter's great red spot. And this stone contained sufficient inexhaustible power to move the factories and industrial plants of half the solar system.
I forgot for a moment that Hol-Dai was listed as psychopathically unbalanced. "Where is this stone?" I demanded.