It had been late on a Saturday afternoon. The free week-ends that the Barrier Base engineers had once enjoyed to take their families for picnics "outside," or to rest and relax, were things of the past, for the work on the Barrier was reaching a critical stage, demanding more and more of the technicians, scientists and engineers engaged in its development. Already diplomatic relations with the Eurasian Combine were becoming more and more impossible; the Barrier _had_ to be built, and quickly, or another more terrible New York City would be the result. Roger had never cleared from his mind the flaming picture of that night of horror, just five years before, when the mighty metropolis had burst into radioactive flame, to announce the beginning of the first Atomic War. The year 2078 was engraved in millions of minds as the year of the most horrible--and the shortest--war in all history, for an armistice had been signed not four days after the first bomb had been dropped. An armistice, but an uneasy peace, for neither of the great nations had really known what atomic war would be like until it had happened. And once upon them, they found that atomic war was not practical, for both mighty opponents would have been gutted in a matter of weeks. The armistice had stopped the bombs, but hostilities continued, until the combined scientific forces of one nation could succeed in preparing a defense. That particular Saturday afternoon had been busy in the Main Labs on the Barrier Base. The problem of erecting a continent-long electronic Barrier to cover the coast of North America was a staggering proposition. Roger Strang was nearly finished and ready for home as dusk was falling. Leaving his work at the desk, he was slipping on his jacket when David came into the lab. He was small for twelve years, with tousled sand-brown hair standing up at odd angles about a sharp, intelligent face. "I came to get you, Daddy," he said. Roger smiled. "You rode all the way down here--just to go home with me?" "Maybe we could get some Icy-pops for supper on the way home," David remarked innocently. Roger grinned broadly and slapped the boy on the back. "You'd sell your soul for an Icy-pop," he grinned. The corridor was dark. The man and boy walked down to the elevator, and in a moment were swishing down to the dark and deserted lobby below. David stepped first from the elevator when the men struck. One stood on either side of the door in the shadow. The boy screamed and reeled from the blow across the neck. Suddenly Roger heard the sharp pistol reports. David dropped with a groan, and Roger staggered against the wall from a powerful blow in the face. He shook his head groggily, catching a glimpse of the two men running through the door into the street below, as three or four people ran into the lobby, flushed out by the shots. * * * * *