Bram seemed to gather himself. "Very well. This must you know.... "I am an agent in the service of the Great World. For centuries we have waged war against the Niss, evil beings who loot the continua. They established an Aperture here, on your Earth. We detected it, and found that a Portal could be set up here briefly. I was dispatched with a crew to counter their move--" "You're talking gibberish," Tremaine said. "I'll pass the Great World and the continua ... but what's an Aperture?" "A point of material contact between the Niss world and this plane of space-time. Through it they can pump this rich planet dry of oxygen, killing it--then emerge to feed on the corpse." "What's a Portal?" "The Great World lies in a different harmonic series than do Earth and the Niss World. Only at vast intervals can we set up a Portal of temporary identity as the cycles mesh. We monitor the Niss emanations, and forestall them when we can, now in this plane, now in that." "I see: denial to the enemy." "But we were late. Already the multihedron was far advanced. A blinding squall lashed outside the river cave where the Niss had focused the Aperture, and the thunder rolled as the ionization effect was propagated in the atmosphere. I threw my force against the Niss Aperture, but could not destroy it ... but neither could they force their entry." "And this was sixty years ago? And they're still at it?" * * * * * "You must throw off the illusion of time! To the Niss only a few days have passed. But here--where I spend only minutes from each night in the engagement, as the patterns coincide--it has been long years." "Why don't you bring in help? Why do you have to work alone?" "The power required to hold the Portal in focus against the stresses of space-time is tremendous. Even then the cycle is brief. It gave us first a fleeting contact of a few seconds; it was through that that we detected the Niss activity here. The next contact was four days later, and lasted twenty-four minutes--long enough to set up the Repellor. I fought them then ... and saw that victory was in doubt. Still, it was a fair world; I could not let it go without a struggle. A third identity was possible twenty days later; I elected to remain here until then, attempt to repel the Niss, then return home at the next contact. The Portal closed, and my crew and I settled down to the engagement. "The next night showed us in full the hopelessness of the contest. By day, we emerged from where the Niss had focussed the Aperture, and explored this land, and came to love its small warm sun, its strange blue sky, its mantle of green ... and the small humble grass-blades. To us of an ancient world it seemed a paradise of young life. And then I ventured into the town ... and there I saw such a maiden as the Cosmos has forgotten, such was her beauty.... "The twenty days passed. The Niss held their foothold--yet I had kept them back.