The short story "Knock" by American science fiction and mystery writer Fredric Brown starts with a description of "a sweet little horror story that is only two sentences long."
"The last man on Earth sat alone in a room. There was a knock on the door..."
This is how the story ends, with one exception, the last person knows who is behind the door, and this person is not the last... Maybe this knock on the door will save doomed humanity.
This is a link to the text of the story: