"It seems you're expected," he said, "third building to the right and they'll take care of you there." Ten minutes later Dynamon stood in the doorway of a large, beautiful room and saluted. The salute was answered by a grizzled, dark-skinned man sitting behind an enormous desk. This man was Argallum, Commander-in-Chief of the Armies of the World. He rose and beckoned to the young soldier. "This way, Dynamon," said he, opening a small door. "What we have to talk about requires platinum walls." Dynamon's face was a mask as he followed the Commander-in-Chief into the little room, but his heart was pounding and his mind working fast. The platinum room! That meant that he was about to learn a secret of the most vital importance to the world. He remembered now, that there was a delegation of Martians in Copia. They had arrived about a week before, ostensibly to carry on negotiations in an effort to avert the ugly crisis that was developing between Earth and Mars. But the conviction was growing among the citizens of Copia that the chief object of the Martian delegation was to spy. It was a well-known fact that the grotesque little men from the red planet had a superhuman sense of hearing that seemed to enable them to tune in on spoken conversations miles away, much as human beings tuned radio sets. They could hear through walls of brick, stone or steel; the one substance they could not hear through was platinum. Hence the little room off the Commander-in-Chief's office which was entirely sheathed in this precious metal. * * * * * Argallum sat down heavily behind a little desk and gestured Dynamon to be seated opposite him. "On the basis of your fine record," said Argallum, "I have selected you, Dynamon, to lead a dangerous expedition. You may refuse the assignment after you hear about it, and no blame will attach to you if you do. It is dangerous, and your chances of returning from it are unknown. But here it is, anyway. "The situation with Mars is growing worse each month. They are making demands on us which, if we accepted them, would destroy the sovereign independence of the World-State. We would become a mere political satellite of Mars. But if we don't accept their demands, we are liable to a sudden attack from them which we could not withstand. They have got us in a military way and they know it. We might be able to stand them off for a while with our fine air force, but if they ever got a foothold with their land forces, then it's good-bye. They have a new weapon called the Photo-Atomic Ray against which we have absolutely no defense. It's a secret lethal ray which far outranges our voltage-bombs and which penetrates any armor or insulation we've got."