"And then one day we witnessed an explosion on Earth's surface--an explosion different from any of the countless ones before it. That explosion was the first man-made release of atomic energy--a process we had known how to bring about for ages, but one we would never use. For we have learned the secret of limitless power without the transformation of mass into energy. Your way is the way of destruction, Martin Kirk; ours is exactly the opposite. "For the first time, the leaders of Mythox knew the meaning of fear--fear that, once Earth's scientists had found the secret of nuclear fission they would go on to the one extreme forbidden throughout the Universe itself. "And so we acted. Not in the way your people would have acted were the situations reversed. For we were still determined that there would be no intervention on our part in Earth's affairs--and that is still our way, just as it must always be. But there must be one exception to this rule: no one on Earth must be allowed to blunder into the extreme I mentioned a moment ago." * * * * * Tamu, overlord of Mythox, paused to drink from his glass and to cast a speculative glance at the stolid face of Martin Kirk. He might as well have studied the contours of a brick wall. "The road to that blunder had been opened the day your learned men first split the atom. If they persisted down that path, it was bound to follow that they would attempt the thing we feared: the splitting of hydrogen atoms--the hydrogen bomb, as you call it. "We know what that would mean: a chain reaction that would wipe out an entire galaxy in one blinding flash. _Our galaxy_, Martin Kirk--yours and mine! Do you have any thought at all on what that means?" The question was rhetorical; even before Kirk could shake his head, the overlord pressed on. "Mythox and Earth are two grains of dust on opposite sides of a galaxy--a spiral formation of stars and planets 200,000 light years wide and 20,000 thick. Between us lie countless other worlds, a vast number of them supporting life--not always, or even often, life as we know it, but life nonetheless. "There is not one of those worlds, Martin Kirk, we do not know as thoroughly as we do our own. Fortunately for our purpose only a relative few have progressed along a line which can lead to danger for the rest. Yours is one of those which has--and that is why we of Mythox have taken a well-masked place in your affairs _so far as they relate to nuclear physics_. "Every scientist of your world, male or female, is constantly under the eye of a Watcher. These Watchers are members of your own races--people we have enlisted in the fight to save not just their world or mine--but millions of worlds.