John Fuller, designing engineer, helped the two men to build a ship which used the drive in order to have a weapon to seek out and capture the mysterious Air Pirate whose robberies were ruining Transcontinental Airways. The Pirate, Wade, was a brilliant but neurotic chemist who had discovered, among other things, the secret of invisibility. Cured of his instability by modern psychomedical techniques, he was hired by Arcot to help build an interplanetary vessel to go to Venus. The Venusians proved to be a humanoid race of people who used telepathy for communication. Although they were similar to Earthmen, their blue blood and double thumbs made them enough different to have caused distrust and racial friction, had not both planets been drawn together in a common bond of defense by the passing of the Black Star. The Black Star, Nigra, was a dead, burned-out sun surrounded by a planetary system very much like our own. But these people had been forced to use their science to produce enough heat and light to stay alive in the cold, black depths of interstellar space. There was nothing evil or menacing in their attack on the Solar System; they simply wanted a star that gave off light and heat. So they attacked, not realizing that they were attacking beings equal in intelligence to themselves. They were at another disadvantage, too. The Nigrans had spent long millennia fighting their environment and had had no time to fight among themselves, so they knew nothing of how to wage a war. The Earthmen and Venusians knew only too well, since they had a long history of war on each planet. Inevitably, the Nigrans were driven back to the Black Star.[A] The war was over. And things became dull. And the taste of adventure still remained on the tongues of Arcot, Wade, and Morey. [Footnote A: See "_The Black Star Passes_", Ace Books, F-346.] I Three men sat around a table which was littered with graphs, sketches of mathematical functions, and books of tensor formulae. Beside the table stood a Munson-Bradley integraph calculator which one of the men was using to check some of the equations he had already derived. The results they were getting seemed to indicate something well above and beyond what they had expected. And anything that surprised the team of Arcot, Wade, and Morey was surprising indeed. The intercom buzzed, interrupting their work. Dr. Richard Arcot reached over and lifted the switch. "Arcot speaking." The face that flashed on the screen was businesslike and determined. "Dr. Arcot, Mr. Fuller is here. My orders are to check with you on all visitors." Arcot nodded. "Send him up. But from now on, I'm not in to anyone but my father or the Interplanetary Chairman or the elder Mr. Morey. If they come, don't bother to call, just send 'em up. I will not receive calls for the next ten hours. Got it?" "You won't be bothered, Dr. Arcot." Arcot cut the circuit and the image collapsed.