"Back, Gregg! Get back!" I could barely hear his whisper. We crouched as far back into the doorway as we could get. I was armed. My official permit for the carrying of the pencil heat ray allowed me always to have it with me. I drew it now. But there was nothing to shoot at. I felt Snap clamping the grids on my ears. And now I heard something! An intensification of the vague footsteps I had thought I heard before. There was something following us! Something out in the corridor there now! The corridor was dim, but plainly visible, and as far as I could see it was empty. But there was something there. Something invisible! I could hear it moving. Creeping toward us. I pulled the grids off my ears. Snap murmured, "You've got a local phone?" "Yes. I'll get them to give us the street glare!" I pressed the danger signal, giving our location to the operator. In a second we got the light. The street in all this neighborhood burst into a brilliant actinic glare. The thing menacing us was revealed! A figure in a black cloak, crouching thirty feet away across the corridor. Snap was unarmed but he flung his hands out menacingly. The figure, which may perhaps not have been aware of our city safeguard, was taken wholly by surprise. A human figure, seven feet tall at the least, and therefore, I judged, a Martian man. The black cloak covered his head. He took a step toward us, hesitated, and then turned in confusion. Snap's shrill voice was bringing help. The whine of a street guard's alarm whistle nearby sounded. The figure was making off! My pencil ray was in my hand and I pressed its switch. The tiny heat ray stabbed through the air, but I missed. The figure stumbled but did not fall. I saw a bare gray arm come from the cloak, flung up to maintain its balance. Or perhaps my pencil ray had seared his arm. The gray-skinned arm of a Martian. Snap was shouting, "Give him another!" But the figure passed beyond the actinic glare and vanished. We were detained in the turmoil of the corridor for ten minutes or more with official explanations. Then a message from Halsey released us. The Martian who had been following us in his invisible cloak was never caught. We escaped from the crowd at last and made our way back to the _Planetara_, where the passengers were already assembling for the outward Martian voyage. II I stood on the turret balcony of the _Planetara_ with Captain Carter and Dr. Frank, the ship surgeon, watching the arriving passengers. It was close to the zero hour; the level of the stage was a turmoil of confusion. The escalators, with the last of the freight aboard, were folded back. But the stage was jammed with incoming passenger luggage, the interplanetary customs and tax officials with their x-ray and zed-ray paraphernalia and the passengers themselves, lined up for the export inspection.