Imagine all human beings swept off the face of the earth, excepting one man. Imagine this man in some vast city, New York or London. Imagine him on the third or fourth day of his solitude sitting in a house and hearing a ring at the door-bell! (Ponkapog Papers, 1904)
Fredric Brown condensed this text to "a sweet little horror story that is only two sentences long." Knock then goes on to elaborate on those two sentences and build a more complete plot around them.
It was published in the December 1948 issue of Thrilling Wonder Stories. There have been three different radio adaptations (Dimension X, X Minus One and Sci Fi Channel's Seeing Ear Theatre). The story was reprinted in The Best Science Fiction Stories: 1949,
The first three lines are a complete story by themselves:"The last man on Earth sat alone in a room. There was a knock on the door..."
The Zan have killed off all life on Earth other than pairs of specimens for their zoo of exotic Earth fauna. Walter Phelan is the last man on Earth, but Grace Evans, the last woman, is not overly impressed with him and maintains her distance.
The Zan, who are immortal, become disturbed when, one by one, the other animals begin to die. They turn to Walter for advice. He tells them that the creatures have perished from lack of affection, suggesting that they pet the survivors regularly to keep them alive. He demonstrates with one of them. When the Zan begin to die, they depart the planet in fear. It is then revealed that the creature Walter advised them to pet was a poisonous snake.
Then Walter discusses the future of the human race with Grace. She is shocked by his proposal and leaves as he intended to use the Zan technology left behind to create "the master race". The narrative then ends as it began:"The last man on Earth sat alone in a room. There was a knock on the door..."