Rolling along the echoing sunken road, Calhoun saw, just once, a ground-car in motion on a far-flung, cobwebby bridge between two tall towers. It was high overhead. Nobody in it would be watching grubby commerce-roads. The whole affair was very simple indeed. Calhoun brought the car to a stop beneath the overhang of a balconied building many stories high. He got out and opened the gate. He drove the car into the cavernous, so-far-unused lower part of the building. He closed the gate behind him. He was in the center of the city, and his presence was unknown. He climbed a new-clean flight of steps and came to the sections the public would use. There were glassy walls which changed their look as one moved between them. There were the lifts. Calhoun did not try to use them. He led Murgatroyd up the circular ramps which led upward in case of unthinkable emergency. He and Murgatroyd plodded up and up. Calhoun kept count. On the fifth level there were signs of use, while all the others had that dusty cleanness of a structure which has been completed but not yet occupied. "Here we are," said Calhoun cheerfully. But he had his blaster in his hand when he opened the door of the laboratory. It was empty. He looked approvingly about as he hunted for the storeroom. It was a perfectly equipped biological laboratory, and it had been in use. Here the few doomed physicians awaiting the city's population had worked desperately against the plague. Calhoun saw the trays of cultures they'd made--dried up and dead, now. Somebody had turned over a chair. Probably when the laboratory was searched by the invaders, lest someone not of their kind remained alive in it. He found the storeroom. Murgatroyd watched with bright eyes as he rummaged. "Here we have the things men use to cure each other," said Calhoun oracularly. "Practically every one a poison save for its special use! Here's an assortment of spores--pathogenic organisms, Murgatroyd. One could start a plague with them. And here are drugs which are synthesized nowadays, but are descended from the compounds found on the spears of savages. Great helps in medicine. And here are the anaesthetics--poisons, too. These are what I am counting on!" He chose, very painstakingly. Dextrethyl. Polysulfate. The one marked inflammable and dangerous. The other with the maximum permissible dose on its label, and the name of counteracting substances which would neutralize it. He burdened himself. Murgatroyd reached up a paw. Since Calhoun was carrying something, he wanted to carry something, too. They went down the circular ramp again. Calhoun searched once more in the below-surface levels of the building. He found what he wanted--a painter's vortex gun which would throw "smoke rings" of tiny paint-droplets at a wall or object to be painted. One could vary the size of the ring at impact from a bare inch to a three-foot spread.