Doorway to Destruction

It was the Doorway to Earth's Destruction. And Kelvin Martin, the only man who could lock it—had lost the key.
Old Kelvin Martin strained futilely against the rope that held immovable his thin wrists. A crimsoned bruise raced across his forehead where Vance had slugged him with a heavy hand.
Don't be a complete fool, Vance! he said harshly. That machine can't bring you anything but trouble!
The scientist's burly assistant glanced wearily up from where he coupled heavy batteries in series at the rear of the glittering machine that entirely filled one corner of the windowless room.
Shut up, he said tonelessly.
Kelvin Martin sagged back in sheer futility, felt a deadly numbness creeping through his extremities from the tightness of his bonds. He watched the other out of eyes faintly fearful and desperate.
I'll make a deal, Vance, he said finally. I've got about eight thousand dollars in the bank; free me, don't try to use that machine, and the money is yours!
Jon Vance's laughter was brittle and scornful.
Eight thousand! he sneered. Hell, I've seen those snapshots you brought back! Any one of those gems the 'other people' wore would bring that. And I intend to bring back all I can carry!
Kelvin Martin shivered, remembering the restless cruelty that had lain in the creatures he had found with his machine. There was still a dull ache along his ribs where a needle-like ray of terrific energy had seared.
They aren't human, Vance. He tried to speak quietly, endeavored to drive his point with impersonal logic. They are of a fierceness and cruelty such as you couldn't comprehend. And with their superior weapons, they'd subjugate the entire world in a matter of days.
Hooey! Jon Vance spat insolently, patted the .45 automatic at his hip. I think I might do a bit of subjugating myself.