Nobody Saw the Ship

Nobody Saw the Ship by Murray Leinster - It was only a tiny scout ship from somewhere beyond the stars; only one alien creature occupied it. But the ship's mission spelled life to its fellow creatures and death to all living creatures on Earth. And against the super-science of the raider stood one terrified old man and his dog.... The landing of the Qul-En ship, a tiny craft no more than fifteen feet in diameter, went completely unnoticed, as its operator intended. It was armed, of course, but its purpose was not destruction. If this ship, whose entire crew consisted of one individual, were successful in its mission then a great ship would come, wiping out the entire population of cities before anyone suspected the danger. But this lone Qul-En was seeking a complex hormone substance which Qul-En medical science said theoretically must exist, but the molecule of which even the Qul-En could not synthesize directly. Yet it had to be found, in great quantity; once discovered, the problem of obtaining it would be taken up, with the resources of the whole race behind it. But first it had to be found.