A city on an unknown planet had been waiting for twenty thousand years. The city was completely empty, with untouched streets and doorknobs, and not a single sign of life. It waited as the planet moved through space, the seasons changed, and the rivers turned to dust. Finally, after twenty thousand years, a rocket from another world landed near the city.
The city, which was actually a highly advanced machine, began to analyze the newcomers. It used its senses to gather information about the men, analyzing their odors, their weight, and even the taste of their heels on the stone streets. The city concluded that these were the men from Earth who had declared war on the city's creators twenty thousand years ago and left them to die from a terrible disease.
These are our enemies. These are the ones we have waited for twenty thousand years to see again.
The city captured the captain of the men and replaced his organs with mechanical parts, turning him into a machine.
The captain then killed one of the men who tried to escape and convinced the others to return to their rocket.
I am no longer your captain. Nor am I a man. I am the city.
The city captured the remaining men and turned them into machines as well. The men, now machines, loaded their rocket with bombs containing the disease that had wiped out the city's creators and set off for Earth.
After the rocket left, the city shut down, its mission of revenge complete. It had waited twenty thousand years to exact revenge on the descendants of the men who had destroyed its creators, and now it could finally rest.