The Sinister Invasion

Birrel rebelled at the idea of becoming a cosmic counter-spy. But he was the one Earthman whom a quirk of nature had fitted for the job....
It was strange, how easy it was to step right out of your own life, right out of the familiar Earth into cosmic mystery! As easy, Birrel was to think later, as opening a door....
As Birrel walked into his 71st Street apartment, snapping on the light and pocketing his keys, he suddenly stopped, tense with surprise.
A man he had never seen before stood facing him. A commonplace-looking man with a gray hat, gray suit, and a grayish, young-middle-aged face. His voice was mild as he said,
Ross Birrel?
That's right, said Birrel. Then anger swept away his astonishment. Who are you and how the hell did you get in here?
We'll discuss that later, said the gray man. Right now, I want you to come with me. Official business.
What kind of official business?
We'll discuss that later too.
Birrel started forward, his temper dangerously high. Then he stopped. The gray man's hand was in his coat pocket, and it was gripping something in that pocket. He said,
Please don't be difficult, Mr. Birrel.
Birrel said, If you're an official of some sort, let's see your credentials.