Over the eastern horizon, a gleaming sliver in the sunlight, thundered the ZX-1, straight for the array of the Black Fleet. Only a few men were aware of the drama-fraught message which had come down from her radio cubby, but her growing shape commanded the eyes of every sailor and officer alike who had time to watch. A few telescopic sights were trained on her as she bellowed ahead; the keen old eyes of a very perplexed and puzzled admiral were at one of them. "Two planes hanging from her rack," he muttered, half to himself and half to the officers standing around him. "Both Navy. Say, they're dropping off! Not coming this way, either. Going northeast. Fast, too. Can't see 'em any more.... Those men getting up from the _Saratoga_? Good. We'll find out something soon. Here she comes!" Closer and closer roared the dirigible. Two planes from the _Saratoga_ were swooping up to enter her rack, but the other two planes that shortly before had been suspended from it were gone--already vanished into the northeast. "Don't understand this at all!" said the Admiral of the Black, or Pacific, Fleet of the United States Navy. * * * * * Things had broken well, Chris Travers considered. He had only wounded the invisible raider; but, luckily, had wounded him badly, so that, evidently, just one object was in the man's mind: to get back to where he came from, to where he could find help. He seemed oblivious of the scout that was following behind at the full speed of its mighty rotary motor, following him to his base, wherever it was. "Just as well I didn't kill him," Chris muttered. The rush of wind had cleared his brain; his faculties were steady and normal. Not so with the man in the plane he pursued. It was flying crazily, but clinging to one course, nevertheless--into the northeast, towards land, some two hundred and fifty miles over the horizon. The great silver shape of the ZX-1, barren, now, of life, dropped away, speeding ever due west; the hazy dots and blur of smoke which denoted the motionless Black Fleet vanished. But Chris was in contact with the fleet's flagship once more, through the compact radio-telephone set of his scout. As he flew, his eyes fixed steadily on the plane ahead, he was rapping into the microphone the story of what had happened. He told of the invisibility of the strange marauder, of how accurately he had judged the time of his raids; of how he, Chris, had managed to prevent the destruction of the ZX-1. "He uses a tremendously expansive gas resembling carbon monoxide," he went on. "It seeps into every cranny of the dirigible, killing everything. The crews got no warning; they didn't know what was happening; couldn't see him! Well, I managed to wound him on the ZX-1. He beat it. I'm following him. If he lasts out, he'll go to where he came from, and we'll find out who's in back of all this. Let you know where his base is soon as I get there. Keep listening. Okay? Right; signing off."