Here's a link to the full story right here, and also here's the full story. It's called "Knock":
http://en.wikipedia.org/wiki/Knock_%28short_story%29
Since it's on Wikipedia, I'm listing the story here. (If it's still under copyright, then I'll remove remove it when I wake up tomorrow morning, unless I die in my sleep of course.)
The last man on Earth sat alone in a room. There was a knock on the door...
Don